Other users advice about traveling, relaxing, and exercising is spot on, but probably not what you were looking for. Besides these are things you should do for a lifetime, not just a summer.

I know you mentioned working at a research lab but have you tried more academic research? You seem to value learning and mentorship a lot, so it could be a lot of fun to pick a professor and work with them for a summer. Given your background, you could probably convince any STEM professor to work with you, and I would heavily recommend exploring stuff outside your area of expertise.