Thank you notes go a long way alone but gift cards for coffee or a travel mug were very popular when I worked with kids
I did baked good for all the teachers for thanksgiving (~40 hand apple pies) from all the apples we picked at the farms lol, they LOVED it. You can also sponsor coffee or a lunch in conjunction with some other moms and the teachers appreciate it. Make sure to ask the director\principal of the school if the teachers have a favorite place for lunch.
I think food is lovely. We always get gift cards and something from Bath & Body. Our daycare is great about putting out a "teachers' favorite things" document that has all of the teachers favorite scents, hobbies, restaurants, stores, and coffee orders, as well as food allergies. We usually get hand soaps and little Starbucks gift cards for everyone, then we add gift cards to their favorite stores for the main teachers. We make sure to give everyone a picture thank you card so they have a personal message from us thanking them for their hard work and pictures of our daughter on it in case they want it for memories!
My youngest has 2 teachers & 4 helpers & oldest has only 2 teachers. Every year I get everyone a soft throw blanket. A lot of times kohls has them for like $7 each. I also bake some desserts & give them out to everyone with the blanket