Gift cards are always popular here. My older son, who's a PITA to buy for, also loves scratch-offs, so he'll get some of them. I also do things like make-up, lip balm, fancy pens, books, etc.
My oldest is a teacher who swears by Ticonderoga pencils. I found some in bright colors--still with regular graphite--to brighten up her classroom.
Younger DD loves post-its, highlighters, and dry-erase markers, so some of those for her.
DS25 also likes a specific type of pen--he's getting some of those
DS17 is traveling a lot this summer--he's getting a fanny pack and a coin purse, among other things he'll appreciate for the trips. He already got our leftover euros for Christmas.
I usually get each child a funny/sarcastic t-shirt. Try
pawsoinate.com if you like personalized dog/cat ones.
whatonearth.com is another source for funny ones.