Having been a classroom teacher for four years, and having many teacher-friends, I feel highly qualified to address this topic. In my short career I have received gifts ranging in value from $2 to $100 (no kidding, totally unnecessary, but fun!). The value of the gift really isn't important, but here are some helpful (I hope) guidelines for end-of-the-year (or holiday) gifting for teachers:
1) Step away from the apples. Seriously. The possible exception to this would be in the case of a new teacher who has yet to be showered in apple-themed nonsense. Unless the teacher is a self-proclaimed apple enthusiast, it really isn't necessary.
2) Consumable is good. Teachers, like most of us, already have a lot of stuff. You might want to steer clear of anything that will take up shelf-space. Again, unless you know the teacher's decor and tastes really well, avoid anything non-consumable that needs to be displayed in the teacher's home.
3) Gift cards are easy, available in any amount, and can be purchased from a variety of stores. Especially at book stores, coffee shops, ice cream, and video stores, a little can go a long way. $5 is a movie-rental or a really nice cup of coffee, even most of a paperback! If you can/want to spend a little more try the mall, restaurant, movie theater, or store like Target or Kohls. A local spa gift certificate might be nice too, but that's not everyone's cup of tea. Want your money to go back into the classroom? Send a gift certificate to a teacher-supply store. You might not realize it, but there's probably one near you.
4) Candles, lotion, scented soaps, etc. are not a bad idea. If you are going to go that route I suggest getting them from a decent place (do you want the seven piece cherry-scented "spa" set on clearance for $4.50?) and possibly include a gift receipt. I know someone who loves vanilla anything - lotion, candles, etc. Vanilla seems like a mild enough scent, right? But to me too much vanilla makes me feel like a cookie factory threw up nearby. I'm not trying to offend the vanilla-lovers, I'm just saying that scent is a pretty personal thing. I always loved receiving gifts from Bath and Body Works because I use their stuff and because you can take any unused product back and exchange it like it's a gift certificate. I have never had trouble exchanging items their store.
5) I always thought notepads (of the non-apple variety!) and note cards were good gifts, especially for elementary school teachers. I went through both like crazy sending notes home, to other teachers, writing thank-yous, etc. If you don't get school-themed ones then the teacher can use them at home as well. You can also have your kids decorate note cards by folding unlined, 5x7 index cards in half, and buying similarly-sized envelopes from an office store. Have your child decorate the front of each card, pair each with an envelope, and tie a ribbon around a set of eight or ten.