I took 2 18 year old boys last May for a couple of days before we went on to WDW. They swam, walked on the beach, played basketball, rented movies and appeared to have a good time.
I'm sure they weren't bored 'cause I told DS this weekend that we were going to Vero again for a couple of days before WDW and it was his choice whether to come along or meet us later in Orlando. (He's driving home from college, anyway, and is perfectly capable of catching his own plane and joining us). His answer: "Why would I not want to go to the beach?" There you have it.
Of course it probably didn't hurt that there were teenage girls around last May also.