When you get to the island, you'll be bombarded with all kinds of tour agencies trying to get you to do all kinds of things. If you're one for adventure, I highly recommend renting a boat yourself and roaming around the Adriatic Sea! There are dozens of islands within your gas tank's capabilities and views to die for. For 400 kuna ($58 USD as of May 2015) you can rent a small 6 hp motor boat all day! And you can pack on up to 6 people. Yes, it's that cheap. We opted for the 25hp motor for 800 kuna (about $116 USD) and split it 4 ways. Less than $30/person to drive a boat around to cliffs, coves, beaches— anything. Talk about a good deal.
Where? We found the cheapest rates from one of the small kiosk-like stands right near the main harbor. Pro tip— do a lap around the main area first to ask for rates, boat size, and engine power. We found they only vary by about 50-100Kn ($7-$15 USD) but hey, that's enough for a couple beers and a pizza.
Pro tip 2— go to the local Konzum nearby (just off the main harbor) to stock up on beer and snacks. You can buy stuff right on the harbor, but prices are about 50% higher.
It's the same price all day, so go early. You can go to an island and post up for 4 hours and still have 8 hours of cruising. We rented at 2pm and brought it back at 8pm, and still found it WELL worth it.