What classes you can take over the summer depends on where you want to take them. I would be careful with taking too many though. First of all because they are usually condensed and thus intensives. Secondly because its summer and some degree of relaxation is important...you dont want to go into the next school year already burned out. I took 2 semesters of physics with a lab one summer and this was enough work for me. I wouldn't recommend taking more than 2 classes at a time in a summer session. From my experience with it, a lot of schools have two 6-week sessions, in which you cover a full semester's worth of work.