That being said the kind of person who is going to spend the rest of their life doing medicine can find a way to do a little reading without ruining their summer or "stressing out." If anything I find those transition periods the best for learning because I can focus on what I find interesting, without being tested or having to turn anything in.
So read a bit about whatever you find interesting. Like ECGs? I recommend Mattu's "ECGs for the Emergency Physician."
Though if you really want to get ready for intern year at 80% of the programs in the country, you'd be better off working on your medical spanish this summer.