Unless you're Martha Stewart or have a baseball team-sized crew of crafty people willing to be at your beck-and-call before opening night, chances are you do not have the time, energy, brain-power or creativity to do this. I have always had great intentions to do gifts, but have never actually done it for entire casts. It's hard enough to do cards. In your down time after a show closes and before you start rehearsals for your next one, a cute idea would be to just go to the dollar section of Target and get a little stash together of cute little things so you have them in the future, but even that's stretching it.
someone mentioned brownies/cookies, and if you have any inclination to baking, this is a FANTASTIC idea...particularly during tech break. i find it's always a little scary to bring food on opening night, as a lot of people seem to forget that whole "don't eat in costume" thing.