We don't have one that we do year after year other than taking a picture of them with their books.
Instead every year I find something different to do and surprise them.
This year I took them to Orange Leaf (frozen yogurt). They really really enjoyed that!
I was glad since this year it needed to be low key.
Last year I planned a picnic in our city park and allowed them to play in our Splash Park.
Another thing I do is try to plan a day off 3-4 weeks into school to celebrate. They have been working hard to get into the flow of regular school, and need a nice break. Even with difficulties getting started it is usually due to coming off our Summer schedule and going back to a regular workload. So I always find this a needed day, and we work better when we get back.
Have fun! Let us know what you decide on, it will give me an idea for next year.

Thanks.