The summer before I went to law school (straight out of undergrad) I babysat 8-5, M-F. It was awesome. I took the kids to the pool, carted them to golf lessons, played tag and watched Full House. Perfectly mindless, ridiculously relaxing. It has not hurt me one bit.
Summer after 1L I worked at a small law firm. They did not even ask what I did the summer before law school. All the interview Q's were variations of "Why did you go to law school?" "How do you like it?" "What was your favorite class?" "Has it met your expectations?"
Now, I am interviewing for my 2L summer. Not a soul has asked about the summer before law school because they all ask about my 1L summer.
Do whatever you want the summer before law school - no one will even know. I'd highly recommend a mindless job (or one that you will enjoy without having to push too hard) to relax before you have to really grind when 1L starts. For the summer after 1L, absolutely get a job that is law-related, regardless of pay or how it relates to the law. You'll need something to put on your resume and discuss in 2L interviews, plus the practical experience (research and writing) will be useful.