Many teachers have some form of "Fun Friday" to reward their students for a week well done. Why not make it a school-wide fest?
Various fun activities can be set up across classrooms like mini-carnivals. Students on the same floor may go to another classroom as their Fun Friday Fest choice. This can also allow students in different grades to interact and build leadership and community.
Students can be the ones to devise the games and activities, and perhaps some older students can even manage the game or activity they created.