I like to collect blind-box toys from Japan (from stores like this: http://rotofugi.com/toyscart/pc/viewCatego...p?idCategory=5.) They're vinyl figurines or models that come in a box, and you can't tell which kind of figurine it is until you open it. It's sort of like the toy version of baseball cards: you don't know which cards are in the pack until you open it. Anyway, sometimes I don't like or already have the toy that I get in the blind-box, so I put the unwanted ones in caches. I'd love to find unopened blind boxes in caches, to me, that'd totally make my day.
I also picked up a bunch of small toys (tiny rubber ducks, playing cards, fun band-aids, pencil toppers) at Archie McPhee -- local store here in Seattle, but they're online at http://www.archiemcphee.com. I'm going to Italy at the end of this month, so I'm planning on heading over to a cheapie tourist shop to buy Seattle stuff (magnets, keychains) to leave in caches there.
A couple of weeks ago, I did find Silly Putty on sale. I bought a couple of packages, but I don't know if they'd get all yucky in wet caches. And they're a little too tempting for me not to keep