I agree with this completely.
OP, there are many people who start med school having a limited bio background, including yours truly. They may be a little behind the curve for the first few weeks, but it doesn't take long before everyone is equally in over their heads. I know this advice flies in the face of the typical delayed gratification worldview that the typical premed (and med student!) lives and breathes, but it really is the best advice.
It's not just you who are in this position. I'm right now planning my fourth year med school rotations, and guess what? The residents are giving me the exact same advice to enjoy my last few months of med school that we med students are giving you.
My suggestions:
1) Spend time with your family, especially if you're married and/or have kids.
2) Hang out with your friends. Call people you haven't talked to in a while. Even go visit them if you can.
3) Go to all those local tourist attractions you always meant to go see but never did. Take road trips. Go abroad for a few weeks if you can afford it.
4) Read whatever books (non-medical!) you've been putting off while you were doing your premed stuff.
5) Don't listen to other people, especially your fellow premeds, who insist that if you don't spend all summer studying, then you're going to be at a huge disadvantage this August because you don't have enough bio background. You're not going to be at a huge disadvantage, because *no* premeds have enough bio background. If they did, they wouldn't have to go through the first two years of med school in the first place. Right?
Seriously, you've earned the right to rest on your laurels for a few months. Getting into med school is a big accomplishment, and now is the time to savor the moment. It won't be long before you are neck deep into wondering what the heck you were thinking when you decided it was a good idea to apply to med school.
Congrats again, and best of luck next year.