If you are the blue-collar type:
-Landscaping (but it's hard work). I made about 10gs one summer doing this.
-Roofing (but it's hard work)
-Waiting tables at a popular restaurant
-Any job working for your local town should pay pretty well (but you often need connections for this). I did this one summer and made ~400/week for sitting in a garage eating peanuts/sunflower seeds and watching TV.
Easier money, less pay options:
-Tutoring. People say you can make $50+/hour, it's highly dependent on the area. As a former teacher, where I'm from, you're lucky to get more than $15-20/hour.
-LIfeguard
-Camp counselor
-Golf course
Make sure to balance effort vs. gain before accepting a job. You don't want to be annoyed for chump change the summer before you start medical school. Relax as much as possible, or go for it and rake in some cash. Aiming for the middle will most likely be a waste of time.