For a first timer I would recommend booking thru Carnival. As you become a more seasoned cruiser you will get accustomed to doing things on your own more.
Also by booking thru their website you can pre-pay for your excursions.
The last time I was there we did the dunebuggies to Punta Sur. It was a blast. Of course we ended up at Carlos and Charlies along with half of the ships passengers. Great tourist bar!