I am a daycare teach and have been for many years. I've seen some amazing gifts, and I've seen parents who ignore the fact that we've cared for their child all this time. >.:(
Honestly, a simple heartfelt thank you note would be nice. Besides the AMAZING stuff we see from well off families on occasion (once I got a $50 bill in a card with a thank you!!!) homemade treats are always nice. A little plate of cookies or such. There was one family and I had both their kids come through my class as they got older, man that Mom could bake. I looked forward to christmas every year when I had her kids! yum!
For thanksgiving last year (it was right after my daughter had started at the daycare, my son's been there for 5 years since he was 4 months old) I wrote up two identical thank you notes on large paper, thanking everyone for caring for my kids and welcoming me back to work after maternity leave. Then I filled a basket for each note with cookies, cakes, fruit, and chocolates and left one in the teacher's room and one in the office. Something simple like that makes a huge impression.
Other common and well appreciated gifts are gift cards to book stores, coffee shops, outlet stores (here in Massachusetts teacher love the Job Lot! LOL)
But really, it doesn't matter as long as you show you care and notice their existence. Every year we fawn over the lovely heartfelt notes we get and rumple our faces at the parents who do nothing. Even an amazing gift will be looked at sideways if the note says "to- Mrs. Rumplebottom, from - The smiths". with nothing else to say. I hope that helps.
OH! PS - don't give coffee mugs or candles. The only teacher who doesn't have a million coffee cups and candles is one who just started her career! LOL