To give your group of PCs a real roleplaying challenge, let them encounter a trap. But rather than obfuscate or try to hide it, state plainly and clearly, up front, in all honesty, that it's a trap. Tell them they can see the trap mechanism and it's obvious how the trap is supposed to work, and if they just press this button, or pull that lever, they can see plainly that the trap will be rendered completely harmless. But they can't get past it until they disarm it.

Sit back and watch the fun.

Transcript

GM: ... So. We've been here ten minutes. What are you going to do about this trap?
Han: It's clearly a very elaborate trap.
Chewbacca: ... It's a dead animal on a stick.
C-3PO: I'll see what Admiral Ackbar thinks. Are you there, Admiral?
Admiral Ackbar: {over radio} You've captured my attention.
C-3PO: There's a dead animal hanging on a tree.
Admiral Ackbar: {over radio} Intriguing. I have no idea what that might be.
Chewbacca: Oh for goodness' sake. I disarm the trap.
Chewbacca: Oh. 1.
[SFX]: Sprooiiing!
Han: Aha. The old trap in the meat on a stick triple bluff.
R2-D2: I haven't seen that in a dungeon module.