well, i have been a magician for quite some time now, and i really like the tricks that are completely impromptu.
a couple of examples of these are coin tricks and mentalism effects. most coin effects are gonna work great, however sometimes you might want to use a deck of cards, because you can say you want to play cards, and before you play you say, "hey, wanna see something cool?"
the watch stopping trick is completely 'propless,' as you call it, or in the magic world we call it gimmickless.
if you are looking into coin magic you might want to get "modern coin magic" by Bobo.
of course a really impromptu trick with cards is the, "this, that, and other," trick, look it up on youtube. then look up the videos that reveal how it's done, it's completely amazing, but really simple.
anyways, if you don't mind carrying around a deck of cards you may want to look up stigmata at
www.ellusionist.com
simply the most amazing trick ever.