For a pre-K teacher I would get a gift card. They don't get paid a lot, and would probably appreciate a g.c. to someplace, even dunkin donuts (if they drink coffee or eat donuts!).
In grade school the class moms collect $ to get the teacher a group gift - sometimes a g.c., sometimes a gift like a vera bradley tote, etc. Usually I do something on top of that, like flowers, etc. One year I gave a loaf of bread from a specialty bread store in town. My kids think it's a wierd gift, but when I give it to people they seem to love it.