i'm also a rising junior.
i'm taking classes to boost my sGPA, so you should have more time to do this stuff than i do.
i'm in a volunteer program at the hospital.
also going to the animal shelter whenever i have free time to volunteer (its a really nice option because i can show up whenever i want.. unlike hospital volunteering where i'm on a strict schedule).
i'm meeting with a professor tomorrow afternoon because i might have just gotten a research position with him for the summer and hopefully continuing next school year too.
lastly, shadowing. i haven't done any shadowing at all, but this morning i got a phone call from my neurologist saying i can start shadowing there next friday afternoon.
i don't have anything else super interesting, sorry, but i think this is just the basic stuff, and you can make your summer worthwhile as far as helping out your med-school application.
i'm not studying for the MCAT at all though... i'll be taking it NEXT spring/early summer so it'd be pointless. i'm probably going to start in december or something like that.