This is my favorite itinerary out of all the ones I have done (done this one 3 times already and booked another for 11/28/10)!
In St. Thomas it is always fun to take the skyride up to Paradise point. It is a short walk over to it and you can do this at your own leisure, just don't wait too late to come back down as there is sometimes a line. It overlooks Charlotte Amalie from a great little spot that serves all kinds of yummies inlcuding the Bushwackers (beware taste delicious but creep up on you). You can see the ship from here and get some great pictures, also has a gift shop and some exotic birds.
In barbados the Harrison caves are very interesting (riding in little railcars) and even if it rains you are dry. If you enjoy rum, there is the Mount Gay rum factory near by (we walked but you can also get a taxi to). This is offered through tours but much less expensive to go yourselves to and purchase a ticket for the tour. Tour is about 45 minutes and costs about $7.00 a person and tours run every hour all day from 9:30-3:30 Mon - Friday, Saturday is 10 -4. Great history and we even got a few samples at the end!!
St. Kitts - If you like forts and history the Brimstone fort is pretty interesting and takes you along the island's main road, great views to the fort. A taxi is great for this and will wait for you and even take you nice and early to the Batik studio on the way to the fort if you like. It is worth seeing and how they are made.
I could go on but here are a few things that are easy to do on your own and not too physically demanding if your grandson is young. If you want more exciting there is parasailing in St. Thomas that is unbelievable and a zipline in Barbados that will take your breath away as your soaring through the tree tops.
Have a great time