I'm a fan of Key Biscayne, but I find the Seaquarium depressing. As for South Beach, it's a fine destination, even if you're not into the party scene. In fact, you can spend time in South Beach without even noticing the party scene unless you affirmatively seek it out. (I usually park in the public lot at Collins and 22nd, which provides access to the beach and an easy walk to Lincoln Rd.)
One interested in the arts scene should spend some time strolling around the Wynwood neighborhood (which was the subject of a nice article in the NYTimes this past Sunday). Grab a coffee from Panther or a nibble at Lester's and stroll around the neighborhood checking out the galleries, Wynwood Walls, and the other amazing graffiti art.