As a teacher, I'm often on the receiving end of gifts from students, so I will weigh in with my faves below.
Things I like: gift cards to Target, bookstores, Amazon, Panera, snacks/food
Things I am less crazy about: candles, lotions, gift cards to Starbucks, mugs, things that say "teacher" on them.
My ABSOLUTE FAVORITE things to receive: Handwritten notes from my students -- I keep every single one I receive! Parent emails praising my work that are cc'd to my supervisors! These emails can be an unexpectedly powerful way to help a teacher advance professionally, and they cost you nothing but your time. Few parents think to do this, but the effect of receiving an email like this huge.
For teacher appreciation week we got dd's classroom teacher a $10 target gift card, ghiradelli's assorted chocolates and a thank you card. I'm also a teacher (high school though so gifts are rare). The only thing pp mentioned that she didn't like and that I disagree with is the Starbucks card. But that's just personal preference. I love coffee and free coffee is even better! We also got the teacher a target card for Christmas. That way she can spend it on herself or on her classroom.
I also typically do Target giftcards (Starbucks if I know for sure the teacher drinks it), a card (I ask the kids to tell me their favorite thing about the teacher & I make sure to include that), and sometimes a small treat or flowers or something. This year I don't know a whole lot about their teachers personally compared to last year when their teachers were a little more chatty/transparent about that sort of thing, so Target just seems easiest.
Re: Preschool end of year teacher gifts
Cons: knickknacks, candles, lotions/soaps