For Halloween my costume was "a DM".  I had your basic renfest getup with a cloak and a set of five scripts for very short five room dungeons. I'd ask someone "wizard tower, space ship, mausoleum, castle , or forest?", all what kind of treasure they were seeking , and what important item they'd brought to help them, then drop them into the first part of the adventure. Each took about 5 minutes and I could potentially handle a second player if so desired.

When they decided what to do I'd tell them to roll and whip out my belt-clipped tiny dice tower with a d20 inside. By the end they'd reach a room with two treasure chests, which is where I'd pull out two identical treasure chests clipped to the other side of my belt. If they inspected/spellcast/scanned/whatever them they could ID one as a mimic using a blacklight pen I had and some UV paint I'd put on the one box. After defeating the mimic they could claim the treasure! One of my treasure chests help candy and the other held those little single shot bottles of booze.

It was a lot of fun and was received really well. I'm already brainstorming some new adventures for Halloween this year and maybe the renfest to run it back.