You can book them online in advance if you'd like, from their website. There will be an easy access link on your eDocs when they arrive closer to your sailing date. If you've paid your booking in full you can pre-register and do just about anything you'd like on thier site.
1. Go to www.royalcaribbean.com
2. Put you cursor over the center tab at the top row of tabs on the screen "Before You Board"
3. Click on "Online Registration"
4. Fill in the blanks and you'll be able to print your boarding pass (Set Sail Pass), even before your eDocs arrive.
If you're not paid in full, you'll have to wait. Once you've printed your pass you'll see the links for excursions.