My favorite place is the Outer Banks of North Carolina. It has everything. There is probably 100 miles of beach. You have the popular tourist beaches and then if you like to be away from crowds there is the National Seashore. You ll find tons of fun stuff to do. If you like excitement there are wave runners available, parasailing available, kayaking, hang gliding.Check out this web site, you may find it helpful.www.exploring-the-outer-banks.com
Since it will be in the summer head to NYC and go to those beaches. My other option is that you head up to Marquette, Michigan and enjoy the cold, but fun Lake Superior. If you head down county road 550 and drive for 5 to 15 minutes you can park on the side of the road and find a bunch of hidden beaches. Depending on where you live that can be a very long drive.
Outer banks, North Carolina has great beaches with warm water. You and you friends can split renting a house there for one week. All the houses have hot tubs, pools, volleyball courts with sand, and you should try to get one close to the beach.