Office swaps where you don't know people well, I figure candy is usually a safe choice. If they won't eat it, they'll surely find someone who can. So are Starbucks or Dunkin (dependent on the audience) giftcards.
I send a lot of cards and/or postcards. Ridiculous, topical, just for fun, for holidays, whatever. I'm always getting postcards when I travel and then scribbling a note, a fact, or a drawing on them and mailing them off. I have a stash. I know when I've been lonely it's been really nice just to receive something (not a bill or junk) in the mail.
I've given my babysitting kids books for their holidays. Stories I've enjoyed, stories that remind me of them. I had a coworker whose bigger-kid-relatives had kindles and he'd give them credit to buy ebooks they wanted.
I gave my riding instructor a horse ornament from Target that I'd repainted to look like her horse. I also gave her socks, but those were because I found The Perfect Socks and they needed to be shared So practical gifts I think are often a solid choice - the little things that might be just slightly too expensive/nicer than usual for someone to buy for themselves
My dad spent most of 40 years giving my mother, every year, a bear Christmas ornament. Eventually some requirements needed to be set up so the tree did not collapse, but she was always a difficult person to buy for and this was the one gift it was basically guaranteed she'd like.