excursion in souther caribbean with a 3 month old and a 3 years old

[treste]

We are sailing next friday with the NCL Spirit to Southern Caribbean (Barbados, Grenada, St.Lucia, Antigua and Tortola) with our 2 sons who are 3 months old and almost 3 years old. We didn't book any excursion with the cruiseline and I was wondering if some of you had ideas of excursions that we could do with the kids on these islands. We are planning to do mostly beaches but it would certainly be fun to try something else.

Thank you!:)