Gift ideas for a teacher?

I have twin boys who started pre-school in September. They attend daily from 8:30 am to 2:30 pm. They only have a few weeks left, and I would love to get their teacher (as well as her assistant) a gift....just to say thank you and show my appreciation. They really do a lot of stuff at this preschool, and I am so thankful that my boys got to experience everything. What would be an appropriate gift to get the teachers? I was looking at the personalize keepsake boxes for teachers, but we got them personalized candy jars around Christmas time. So I didn't really want to do that. Would a nice flower bouquet be a good idea? Or do you think teachers would prefer something else? I am so clueless, but I would really like to get something nice for them. I am not trying to "suck up" to the teachers, I just really do appreciate all they have done in the past 8 months. So any ideas would be great!