There are 4 "blocks" during the summer for all mids. Block 0 is between the last exams and commissioning week activities. Block 1, 2, 3 are approximately 4-week chunks for the rest of the summer (think, June, July, August). Each mid will have assigned activities for each block. Most regular mids will not doing anything in Block 0 except standard duties associated with closing out the academic year and getting ready for whatever other stuff they will have.
Rising Youngsters (the current Plebe class) will have one block assigned to PROTRAMID where they will get a sampling of each of the line branches - SWO, Subs, Aviation, Marines for a week or so each. Many of them will also do a YP cruise. Most will get a block of leave to go and do whatever they want (see family, travel, etc.). Some will do internships, some will do NASS detailing.
Rising 2/C (current Youngsters) will have other summer activities. Most will have a specific branch cruise (SWO, sub, aviation), many will get to be detailers to the incoming Plebe class, and other things like internships, etc.
Edited to add: Or, just read what the good CAPT just posted.