Hi Caitlin, this is a great question. I'd love to hear what other teachers have to say about what they've done in the past. I'll let you know if I come across anything that would help you!
One thing I have seen other teachers do is having students create end of year projects in the Sandbox. Students create games and then have a showcase where younger grades come into the classroom and play the game or interact with your students' projects. That's the first idea that comes to mind for me.
Anyone else have ideas?