Hello forum!
I'm working on creating a large module on par with some of the official content WotC has released, designed to take characters from level 1-5. I hope to make this available to the public in the next few months. Much of the story and background is already settled in my mind, but I'm having a little trouble in the imagination department when it comes to traps for the numerous encounters I have planned.
Briefly, the adventures take place in the ruins of a city that was buried under the sands of a desert region, thousands of years ago. Recently, massive sandstorms have swept away the cover, revealing the ancient necropolis. Explorers, dungeons raiders and scholars are flocking to the site to delve its secrets.
Besides planned combat encounters, I have a few environmental hazards in mind that will stymie the would be adventurer, but I'm having a harder time imagining what sort of traps could survive a couple thousand years in this sort of environment. Anything of a magical nature, for sure, could have lasted, but even there I'm sort of drawing a blank. It's worth noting that many, but certainly not all, of the encounter zones take place in dungeon-like environments that weren't collapsed and filled with the overlaying sand over the millenia.
Best I've got so far is fall-away floors with pit traps, as they aren't reliant on complex mechanical parts which might malfunction with the entropy of millenial time. Also, there will be a number of competing NPC explorers who aren't above setting up their own defenses as they look around, so I'm happy to hear ideas for their ilk. I've been consulting the 4e Dungeon Delve book for a few ideas, but haven't latched onto anything yet. Honestly, I'm not even sure what exactly I'm looking for, so anything that fits the criteria above (desert environment, been buried for a couple thousand years) is a welcome suggestion!
One of my own classics is an Anti-Gravity trap, where Gravity is just reversed. You can pair it with spikes/staligmites/etc. on the ceiling, otherwise the fall damage might be sufficient. If you're smart, you can place a special treasure in a place, which you can only reach per trap or other means of defying gravity (or some good rolls by throwing a hook on a rope and climbing up, depending on design). The cool thing about an Anti-Gravity trap is, that it works twice: Up and down! 
The Totally Not a Pit Trap, Swear
(See the posts that follow each comment for people interacting with them and the final reveal.)
I always had a similar issue with traps. As much as I enjoyed Raiders of the Lost Ark it was always - wait, what how is that ancient temple firing darts?
Some of the things I found/came up with
- Animated skeletons dressed in servant's rags who's orders were to maintain the traps for eternity
- Walls that collapse that were holding back loose sand. Not technically a trap, more of an environmental hazard
- Falling rocks attached by wire to doors
- Dried powder/poison traps.
- Lake of poisonus substance (mercury)
- Razor wire at neck level
And of course I think I used something like what iserith points to.
Basic levers hidden behind the walls: you step on a particular tile in the floor. CLICK. From behind the walls, the PCs hear audible groans as of old machinery moving without being oiled ... and a panel falls out of the ceiling behind you. Maybe it gives somebody a headache, or maybe it releases hopelessly-stale air ("poison gas" because no oxygen) or perhaps a swarm of vermin now has access to the corridor.
Keep in mind that if you have Random Encounters, anything that holds the PCs' attention and makes them stay in one place, is a sort of trap. Cover the walls in hieroglyphics and art of what the mummy did while he was a living man. He, like Bilbo Baggins, was usually an ordinary fellow except that once he had an unexpected adventure ! Clue to further adventure (painting of a map?) is placed on the wall, if the PCs can just decipher / transport it in time.
Edit: I almost forgot: Grimtooth's Traps (2nd Ed) have some over-the-top stuff that can be toned down into fun challenges. (Ex: bag of marbles to trip on is also tiny glass cavities full of acid. After rolling underfoot, the acid is shook up and makes fumes when you crunch the marbles underfoot. Or maybe this trap was built by giants and rolls bowling balls down the hallway.)
I think one of my favorite traps would work splendidly here. This one I came up with as a result of players looking for traps so often that I would add traps just because they searched.
Have a stuck door in the area. Require the player to make a strength check if they wanna open it (this destracts from the idea of it being a trap.) Upon the door being opened the ceiling above it collapses. Dex save with half on successful save for the player who opened the door, dex save or no damage for anyone within a certain amount of feet from the door.
This one was a fun one to come up with because the building they were in was ancient, so rather than being a set-trap it was really an environmental hazard due to the age of the place.
I like traps that players choose to walk into.
Picture a hexagonal room with one alcove and 2 mirrors on the wall. There is a 5ft wide pit around the edge of the entire room, save the entrance the PC's came through. It's 20ft deep with wicked spikes and skeletons. The PCs would have to take a running jump to make it over the pit and land "safely" in the alcove - or if they figure it out - through one of the mirrors. Ancient crumbling heiroglyphics on the walls depict just that... figures jumping across into the pit and through the Mirrors. There were 3 mirrors a long time ago.
The alcove is a 10' by 10' empty bleached (clue) dusty stone area. The two mirrors shimmer ever so slightly, and it's surface acts like diffused mirror. It catches and reflects torchlight especially well. If a light is shone directly into the mirror, twinkling stars can be seen.
Ancient writings on the walls tell of three very different areas that the mirrors potentially take you to. The alcove translates to "The Vault of Stars", the mirror on the left "The Hidden Depth" and lastly "The Secret Stair". If the PCs look into the pit in front of either mirror they find the skeleton of an adventurer impaled on the spikes. It's still weating its leather backpack, inside of which is a crumbling journal that goes on at length about each mirror's destination.
When a PCs jumps across, there is a liquidy GLOOP noise, the mirror ripples wildly and then the image of the PC gets fainter and fainter (yet still visible). The PCs movements also seems to slow down. (It's reminiscent of falling into a black hole. - GM descriptive note)
The PC has of course just voluntarily jumped bodlify into a gelatinous cube.
The glinting stars are pieces of coin, metal and jewelry that refract off the light.
The trap works well if the PCs think they are solving a puzzle. So, knowledge rolls on where the portals may go, who built them, what the journal's margin notes mean, go along way to getting a PC to voluntarily jump across the pit. Having two mirrors gives the players the illusion that there is a right choice, there isn't. Don't tell the volunteer immediately what is occuring. See what the rest of the party does first. Record the number of rounds, make private rolls, for all intents and purposes the volunteer is fighting for his life in the acidic jelly. Note that sound doesn't travel through the gelatinous cube. The party will most likely tie a rope to the volunteer. Dissolve the edge of the rope in a number of rounds that increases the tension, or likelihood of another player following. If the PC made preparations to tug on the rope if it's a trap, he won't be able to do so on the first round, but give him a STR check on subsequent turns. Reward the cautious player.
Once introduced a trap that was so diabolical, it'll make even a LG person laugh...
Imagine your basic, 13 in a dozen, trapdoor. But this time, instead of sharp pointy things or a carrion crawler at the bottem, place a magical portal on the floor!
Then, on the ceiling above the same trapdoor the exit of previouse said portal. Problem here is, when someone falls trough for the first time, he/she has only a split action to figure it out! Becouse basic physics tells us that the more time we fall, the faster we go untill you reached terminal velosity! A basic human without armour has one of almost 200km/h or 120M/ph!
So the unfortunat person cought in the trap will eventually keep on falling. And each time he/she goes through the portal, he picks up speed (just basic altitude) comparing to the amount of space between the portal and exit of the portal. If there 10 feet apart, 5 times through equals a 50ft drop in just a bit more than one second. In one second a person falls 9.8 meter, or 32 feet. The next second the person has travelled 39.2 meter, or 128.6 feet.
And if a person reaches out or tries to step out, it's like getting hit by a stationairy object while you travel with 200km/h.
Can't remember how the party rescued there friend, but i'm sure there are a lot of options someone can try.