I went on an Eastern Caribbean cruise last March and of your destinations, the two I hit were Tortola & St. Maarten - really lovely all told.
Tortola
Easily the best stop of our cruise as my friend and I decided to take the "Best Of" excursion offered by our ship - it was essentially a guided tour of the island, followed by an afternoon at a beautiful beach with lunch and a free drink included. (Btw, the free drink was tiny but for $1 extra you could add as much rum to it as you wanted which let my friend and I get blitzed really cheaply.) The beach was beautiful with all the leaning palm trees and gentle water you could ask for without being too crowded and while you had to spend a couple bucks for chairs & towels it was worth it cuz you really did want to lounge when you weren't swimming.
The tour was really cool too if you like learning a bit about the island itself - we went up 900 feet in less than 20 minutes of hairpin turns and learned a bit about the culture - including that they have cows all over the place wandering free and waiting to be made into steaks considering the way the locals drive.
The views were breathtaking too. If you want to get really blitzed on bad alcohol take one of the party bus tours - they did much the same we did save with rum shots at stops and no beach (we couldn't get into that one) but had a better time according to the folks we talked with who did.
Also if you want to do some souvenir shopping, there's a nice little tourist market set up just as you get away from the docks (was in walking distance from my ship's gangway). I liked looking thru there but the best place to stop was at a spice shop called Sunny Caribbee - I got some tasty Jerk Spice & BBQ Spice there as well as gifts for family there (my only regret was in not buying more there for myself).
St. Maarten/Ste. Martin
Depending on where you dock, there is generally a taxi queue at the dock with established and posted rates for trips to the main tourist areas of the island.
We went for lunch over on the French side in Marigot for something like $20 USD and did some strolling around after for some shopping - I felt it was expensive but nice area to look around at. After a couple hours we headed back to Phillipsburg and one of the beaches at a resort where for a few dollars each we got a towel and umbrella and hung out there before returning to the ship.
Beautiful island all told - you'll no doubt enjoy it, although I have to say of the places I saw, Tortola was my favourite and Phillipsburg was one or two down on the list. If you really want to shop for electronics tho, it's St. Thomas in the US Virgin Islands where you want to be - my friend got a great deal on an Ipod Nano & Sony digital camera while we were there (the place is totally a tourist hole but it was good shopping).
Mostly just enjoy and sometimes it is better to go off on your own like we did on St. Maarten but there were some really good (& cheap) excursions to be had that we liked too and I would've hated to miss.
Eastern Carribean Cruise
