Dominica is not a resort-type of island, in fact on our tour we didn't even see a beach! For kids, you could do the river tubing, that looked like fun. We booked a tour with Reynold (reynotours@hotmail) and he took us to the rainforest, emerald pool, trafalgar falls, all the 'must sees' on the island. If you can stand 4 hours in a van on twisty-winding roads, you should look into a tour - it's a very pretty island.
In Aruba we took the public bus ($2 r/t) to the beach at the Holiday Inn. There they have a pier where you can book tours, rent snorkel gear, all the water sports you could want. If you go closer to the Playa Linda next door there is a nice little beach bar that has happy hour from 12-5 and serves big frozen drinks at 2 for one prices. Their food looked good, but we didn't eat there. Also, I totally recommend the Kukuunooko Bus (adults only). Definitely an experience not to be missed! We booked on line for $55 each, vice $79 they paid on the ship.