I did Microbiology including the lab portion over a 5 week summer session. I had class 5 days per week, and I actually enjoyed the fact that it went by so quickly. It forced me to study daily, and I retained the information from day to day easier. I ended up with an A+.
If you have to do a science class during the summer, I think Microbiology is one of the better choices. While it is a science, it is the most fun class I have ever taken. Most other people enjoy it as well.