That's a tough call! Those are all great ports of call with a ton of things to do.
In St. Thomas I'd suggest the cable car to Paradise Point. The view is awesome and how long you stay is up to you, so you'd have lots of time for shopping. Also, if you like the beach, Megans Bay is worth the trip provided there aren't more than 3 or 4 ships in port, then it might be too crowded.
The ship will dock at Fredrikstaad in St. Croix but you can get a bus to Christainstaad-about 1 hour and $12 return I think. Christianstaad is a gem! Beautiful harbour, quaint streets with boutique-type stores and wonderful cafes and restaurants. And don't miss the Fort!
St. Kitts has a new shopping area right off the dock-Port Zant I think. And a trip to Fort Brimstone is worthwhile-more magnificent views.
Nelsons Dockyard in Antigua is a nice excursion but the island is known for the beaches so we nearly always do some type of beach or boat thing.
Only been to St. Lucia twice-did an island tour with lunch which was pretty good and a catamaran snorkeling/beach excursion which was fantastic. But I think the thing to do is go see the Pitons!
Whatever you decide, have a blast That's a great itinerary and the Summit is a beautiful ship.