The Dumbest Trap I Still Use

[Edit, Feb 12: Sleepy writing was bad writing. Now edited for spelling and clarity. /Edit]

The simplest version consists of a couple-three ropes (one for your feet, one or two for your hands) suspended across a chasm full of spikes; on the other side the path turns out of view, and only once across can you see that it leads down to the pit. Crossing isn't terribly difficult except that you have to attempt it again on the way back (and again every time you fall).

This is from years and years ago when my group was in a rut where they expected every challenge they encountered to be mandatory and mission-relevant. I usually include it in a biggish complex where there's other opportunities for curiosity to pay off, but I do sometimes like it as filler in smaller sites too, especially if the PCs can fly or have video drones they want to show off.

I've not tried this yet in 5e. It'll be tricky to get the DC right, [s]ince a Rogue doubly proficient in acrobatics can roll ten or more point higher than the other PCs. Maybe a low-middling roll means you fall on your first crossing and a high-middling roll means you don't fall until the [return] crossing? Then roll anew after you fall?

I like the rope bridge as a way of cluing the PCs into an NPC party being ahead of them. The NPCs made the first rope bridge, plus a second rope bridge going further, except they cut the second one from the far side so that its ropes now hang down in the middle (forming the path up from the pit).

Besides spikes I've used sleeping lions, rocky rapids, and terracotta soldiers holding spears - anything to make it less likely the rest of the party can just pull their friend back up. A player on Uncle Matt's channel related the idea of a pit which narrows to where a falling character takes damage and gets stuck and then gets attacked by otherwise easy to evade/defeat vermin.

Haven't tried a swarm of living spikes. Got to integrate it with volcano sharks and sinkholes somehow.

Ideally the chasm shows enough other dungeon features (tunnels, balconies, bridges, courtyards, chimneys, bulges, buttresses, etc) for the players to deduce the overall shape of the dungeon and plan their movements more effectively. Also fun [to put] murals or other optional clues [on the same wall the PCs start from, so they can't see it unless they cross and then look back].