jenna lee

You shouldn't have any issue.  I'd suggest that you book the other two nights as soon as you get on board so you'll have more times to choose from.

Being active the first thing I thought of was a suggestion @lauraodonnell86 made on these boards about Pinel Island.  You may want to look into exploring it and doing it the way that Laura did with renting a kayak to get there!
 

@jenna lee, if you and your boyfriend would like to try something unique, and are a bit active (and a water sport), I highly recommend the America's Cup Racing Experience.  They have a bunch of real (retired) America's Cup racing yachts that they do a mock race, with guests like yourself helping crew the actual boats.  Real fun (and can be a bit of a workout depend on which station you are assigned).  I have done it twice now, including a few weeks ago, and had a great time (and the boat I helped crew won our race).
I booked through Royal on the cruise planner, but I believe you can also book directly with them

Hi @jenna lee, welcome to the forums!
I'm going to give my usual shill for Soualiga Destinations in St. Maarten. Great group of people, fantastic boat ride and snorkeling trip around the island, worth every penny we spent. You can see my review of them here.

Welcome!
St. Maarten / Saint Martin (you can access both): Maho Beach is supposedly fun, never been personally, on my list. We went to a different beach, one of the "nude" ones (where no one was nude) and it was - eh, a beach. Beautiful, but again just a beach.
Adventure on Anthem: Ripcord by iFly, I loved that. (Upset it's all booked for my upcoming Anthem, hopefully my suite concierge can hook me up.) We also enjoyed the North Star. Personally never done but the FlowRider (surf simulator) looks like fun.
It's an awesome ship, you'll find lots of adventure onboard.