Last summer I volunteered at two different places (maybe around 24 hours a week total) and I finished up studying for the MCAT and then worked on apps. I also had plenty of time to relax. I think you'll be fine not working and doing research, as long as you have a relatively strong application. Most schools ask what you are doing the year before medical school, and not just that summer. As long as you'll be taking classes or working for the fall and spring before med school, I think you can relax a little over the summer.