Def. check out local 1$ stores and see what jumps out at you. The possibilities are probably endless.
I saw someone said something about a card making kit.. that is a great route to go. And easy.. just throw some embellishments in a bag, card-stock and a bit of double sided tape (so you don't get into buying tons of glue) and there you go. Tags are another route in that dept.
What about mini scrapbooks.. they are all over the place and def. at dollar stores.. just cut paper to fit the scrapbooks (cardstock, graphic paper, etc.) and put ribbons, embellishments, stickers, etc. into a bag as well. That is something that takes a bit of time to make (even with all the gear in front of you) and something they could add to when or if they wanted.. and it is easy cheesy to put together. Mostly just paper!
What about small paint by numbers? I see those a lot.. and some are really detailed and cool. Could be a bit messy, but eh.
Personal journals are great too.. bookmark making kit.. hm.. really this list could go on forever. Friendship bracelets are easy to put together too.. make an instruction sheet and all you really need is thread or threading floss. Sachets, anything you can do with tissue paper (b/c it is so inexpensive).. such as cover/decorate a tealight holder, make faux stained glass, yada yada yada. OHH I did this recently for a bunch of friends - I found these great "decorate it yourself" travel mugs at the dollar store and instead of coloring the pattern inside I used it and cut out some really cute and funky fabric and monogrammed each for a dif. friend.. you don't have to get that fussy - maybe pre-cut a few pieces of fabric (if you can find the mugs that is.. they were sort of a one off) and they can pick and choose what they would like, or decoupage them, whatever they would like to do! Good luck finding those, though
I should have bought tons while I had the chance.. *sigh*
Anyway hope this helps