a pot of flowers, a hanging basket or even a bouquet is always nice.
If you are crafty, there are a few quick projects you and your son can make, The first and probably easiest is to have your son paint or draw a picture, or put in a photo of the teacher and your son,- and buy a nice frame for it. The teacher could always use the frame for personal photos or keep it as is for a momento. I also like to take the clear arcylic frames with the magents on the back that are for on the refridgerator and decorate the edges with stickers and mount a photo on cardstock and slide that in.
Another more involved idea is a decorated clipboard. You can google them on Etsy for ideas or go to You Tube to see a demonstration of the basics of how to make one. That was my son's gift to his preschool teacher this year. I covered a clipboard with apple themed scrapbook paper, used stickers to put the teachers name at the bottom, added some dimensional embellishments and clipped on a snapshot of my son and a handwritten note from him thanking her for the year. If you'd like to see, i can email you what i did.
Stationary is always a nice gift, with a nice pen
and honestly gift cards are super wonderful. They aren't boring if you present them right,
a gift card for a mexican restaurant could be attached to a bottle of hot sauce, with a note to Hope your summer sizzles, stuff like that.
There is a blog that i think is called Skiptomylou that has lots of cute sayings for appreciating teachers.
You could buy an "almond joy" candy bar and write a note about how the teacher brought your son "Joy" this year and attach a gift card for someplace fun.
Or some life savers- Teacher you've been a life saver this year don't know what we would have done with out you etc etc.
decorate a pot and put in a gift card to a nursery or gardening center.
Buy a children's book your son enjoys for the classroom and attach a card with a gift card in it for the teacher to buy herself something at a local book store.
maybe these ideas will spark something for you,if not maybe someone else can use them. I think our teachers do such a great job with our precious little ones, I like to really take the time to say thank you. I'm sure with most teachers that it really isn't what you give them it's just that you even thought to do something at all.