Best sandbar or island to go to?

We're a large group of 9 in late 20s going to key West next week. We have a 60ft yacht booked for a 4 hour cruise.

Where would you suggest we go to experience the best, least crowded sandbars or islands or mangroves?

What should we do in this 4 hour excursion? Also, we're looking for a captain, so if any local captains around, please suggest where we could go!