Short/Easy Half Day with kids ages 4 and 7?

Antigua is our first stop of 5 ports in a row... we don't want to get our kids completely worn out on the first day. What would you recommend for something that is fairly easy to access, not incredibly expensive (since we are only spending a half day- pay as you go preferred over AI) and back on the ship by late lunch time for some naps? We thought about Jacqui O's wasn't sure how child friendly this was. Really a little something fun with easy travel and no hassle is all we want. (We wouldn't get off the ship at all but after a few sea days I think they will be ready for land.)