What grade level? Some of the things that I like to do with middle and/or high school after the last concert:
Sight reading, including possibilities for next year. Not like you’re preparing for a competition, but sight reading stuff that is easy enough for them to play well and enjoy. Some people that I know will start pep band music for the fall, especially if they teach younger kids who will be entering that group next year.
Student conductor days- once the first kid volunteers, they all want to try.
Recital days- have kids prepare a solo or chamber piece and give an informal performance.
Music theory or appreciation unit- when my high school kids have extra time at the end of the year, we’ll get in depth about a specific piece, like Rite of Spring.
At the very end, have kids help with refiling music, doing inventory, getting the room all buttoned up for summer.
Have fun. The other up side is that your concert will happen when the kids can still concentrate, and before all of the other end of year craziness starts. The concert will feel more special if it’s not sandwiched between awards night and the sports banquet and every other event.