Summers vary greatly depending on the individual mid and, on the philosophy of the then-Supe. Under VADM Fowler (now departed), mid summers generally consisted of these "core" requirements:
3/C summer -- grey boat as an enlisted, another month doing something professional (YPs, sailing, NASS), one month leave
2/C summer -- ProTraMid (one week each at Quantico, P-cola, New London), one month academics at USNA
1/C summer -- one month leadership (Plebe Summer, YPs, Sailing), one month working as a "JO" either on a ship, sub, or now an "aviation" slot, or BullDog for aspiring Marines, or SEALs for aspiring SEALs, etc.
There are probably as many exceptions to the above as mids.
Some do extra academics during the summer, some go abroad for 2 or more weeks, some do exchange cruises, some do programs associated with their academic dept, some return early for sports, etc., etc.
Most mids get about a month of leave each summer -- not necessarily all at once. And, if you choose to do "extra" programs, you often do that at the expense of leave.
The above may have been tweaked slightly since the summer of 2009, when that was the "agenda." And, as noted, with a new Supe, the emphases may well change again.