You do not have to enroll for the summer session. That you will not work illegally. International students are not allowed to work off campus unless they have been given permission. Please consult the Office of Student Support Services for the guidelines for accepting work. That you will maintain an acceptable academic standing.
Some visas allow one entry into the U.S., some two, others multiple entries.
For example, if your visa is for multiple entries into the U.S., you may travel in and out of the United States as many times as you wish before the specified future date without having to go to a U.S. Consul for another visa stamp. Request a multiple entry visa. You may want to travel to Canada or Mexico for a short vacation.