It's probably a toss-up for me. There's nothing I would actually prevent people from touching (well, there's my collection of dead bugs from a few years back (including a 3-inch dragonfly from the Canary Islands), but that's mainly because they're so fragile, and no-one seems to want to touch them in the first place), but there are certain things I'm careful with when there's other people around.
-My laptop. I'll let plenty of people play games or surf the web on it, so long as I know I can trust them, but I wouldn't want them to go snooping through my files or actually moving it, it cost me over ?1000 (eh, it's a Dell). By extension, my external hard drive, which contains at least that same amount of money in games (and some files I really really really don't want to lose, like photos or partial mods I used to be working on.
-My LEGO. I have more LEGO than I can remember, going back years. I'll let kids play with it if they want to, but I prefer they stay away from models I made/modified myself (like a complicated, bigass helicopter that now has a new superstructure, or the 2'8" submarine).
-My booze. Currently I only have a single 700ml bottle of Jameson's I keep for special occasions, hidden behind a box in a small cabinet, but I hope to add at least a 1l bottle of Flagman vodka to the stash in the near future, next time I go to Russia (which is in June at the latest), and probably more in the future. Oh yeah, and there's still some beer left over from a party I held two weeks ago, but that's just because I'm lazy when it comes to cleaning.