There are lots of posts on this site by seasoned cruisers that should help you. With mom you need access to facilities the teens would probably love the beach so any of the beaches menitoned on this site should be great.
There are just so many options it is really dependent on what you prefer to do.
I guess that would depend on what you like to do. I'll assume that since you're going to the Caribbean, you like beaches, right?
St. Thomas: My favorite beach on St. Thomas is Morning Star Beach. Take a cab to get there, nice beachside bar/restaurant with chair hammocks! . Beach is free. Heard Coki Beach is very very nice but haven't been there. Most scenic beach is probably Magens Bay. Knowing what I know now, i'd go over to St. John to Trunk Bay Beach, usually your ship will offer an excursion there. If you go to Trunk Bay, there won't be much if any time on St. Thomas to shop. Or a snorkel& sail catamaran excursion to St. John. Sailing past the Pillsbury Sound is a really awesome sight! We went on one twice called the Champagne Catarmaran sail & snorkel to St. John. Not sure if RCL offers that tour... We booked it on a Holland America cruise and a Princess cruise and it was titled the same. This cat is only 4 hrs. which will leave you plenty of extra time to shop in Charlotte Amalie. Here's a link to the tour operators site...
If others don't like to snorkel they can swim over to the beach, very shallow with beautiful clear water, gorgeous secluded beach (Honeymoon Beach) Or they can just stay on the Cat and have lunch & drinks. They serve a drink called the Painkiller. Be careful though, limit yourself to 2, that's all you'll need!
St. Martin: My Favorite beach on St. Martin is Orient Beach
Hint: You can catch a high speed ferry to St. Barts for the day, much nicer island. I have been to Orient Beach a few times. Last time we went there we rented a car (do not do this). Got lost all day trying to find a better beach. Heard from many that Orient was the best anyway.
Cococay: Get of the ship early to make sure you get a hammock! Also, there is a secluded area if you go strait across from where the tenders drop you off. Once you've gone strait across, just go down around to the far right. Gorgeous beach down there past the inflated slide.
Which ship are you going on? We did this same trip on the Mariner of the Seas in Jan. What a ship! That was the 2nd time we went on the Mariner, this time we did a back to back, 2 weeks. Here's a link to our page... Our cruise on the Mariner of the Seas 1/06
When I was in Nassau (on my E. Caribbean cruise), I took the shore excursion called Discover Atlantis. It took us to the Atlantis Mega Resort and Casino. It was allot of fun, and very fascinating. The aquarium inside the resort has to be up there amoung the man-made wonders of the world. However, there is allot of walking involved, once the boat drops you off at the pier on Paradise Island, until you reach the Atlantis resort. Therefore, your 70 year old Mom might find all the walking to be too much. Also, if you decided to go, and it is time to head back to the boat that will take you back to your cruise ship; you may encounter some Bahamian men that will insist you go a different way back to the boat pier then what you remember. Just ignore them. They are trying to drum up business for a competing boat company that will charge you more.
On CocoCay, there is basically parasailing, snorkeling, maybe scuba, and Jet Ski rental. Other then that, you could just lay on the beach, soak up the sun, and eat the food in the pinic area (which I remember liking). There is also a much smaller (scaled-down) version of the Straw Market also on CocoCay. From what I remember the souvenirs weren't such a bargain. Basically, CocoCay is a small party-island owned by Royal Caribbean. It is maybe no more than 20 acres in size.
When I was in St. Thomas, I took the Five Star to St. John Shore-ex. It basically consisted of a very nice boat trip onboard the Leylon Sneed . They took us to Trunk Bay in neighboring St. John. There I went snorkeling and saw some of the most beautiful coral reef I have ever seen. The beach was absolutely beautiful too. When we headed back we were offered rum punch (and virgin punch, too). That in addition to what you could get in the snack bar onboard. There was also dancing/limbo and other fun. I had a great time on that shore-ex.
I did not go on a shore-ex while in St. Martin. But I heard really good things about some of the catamarand shore-ex.s there.
We just returned from a cruise where St. Maarten was one of the ports. Our adult children were with us and wanted to try the Segways. We booked that excursion on the ship. It was great! All eight of us participated and it was our favorite excursion. Our ages ranged from 34 to 64, so if you Mom is healthy, she would easily be able to do this and it would be something the whole family could enjoy.
We went on the first excursion of the day. It started like at 8:00 or so (can't remember for sure). We were glad that we chose that because there was no one on the boardwalk and we could zip around without worrying about hitting a tourist!
Tell me more about the Cat tour in St. Thomas. We're thinking of it for Jan 28th cruise. I was disappointed in some of the Celebrity recommendations on our Western cruise so I'm a bit leary. Thanks, Steve Jellison, Manchester, MO
The cat tour to St. John was by far the best sail/snorkel we've ever been on... We were lucky enough to get to do it twice so far about 2 years apart. I think the ships (many offer the same one) call it "Champagne Catarmaran Sail/Snorkel To St. John".
Here's a link to thier site... Champagne Catarmaran Sail & Snorkel To St. John
The serve a wonderful rum drink called the Painkiller. Try one or two, more than that you might have to stagger back to the ship! This tour allows you to swim up to an idyllic white sandy beach with gorgeous coral formations and bright, colorful tropical fish everywhere. You can snorkel in shallow waters if you like near the beach which is nice. The views from the cat on the trip there and the way back are amazing. They stop to snorkel at Honeymoon Beach, just beautiful.
Not sure if I could pick between Trunk Bay and this excurion though, both are totally awesome.
We chartered a 39-food sail in St. Thomas, were served a wonderful lunch, snorkeled and had a wonderful day. Captain is a gal, American who really works to accomodate you. $115 per person with a limit of 6 or 7 people. For an extra $20 per person she arranges to have Wally pick you up dockside and deliver you back at the end of the day. He does a great job of showing you the island while enroute to the marina. Let me know if you want her website. firstname.lastname@example.org