Dominica is a wonderful place. We went there on The Carnival Destiny. Believe this: we also went to St Thomas, Barbados, and Aruba... my favorite: lowly Dominica. The people are wonderful, but poor. It's not Americanized at all, which to me is a great thing. It's an experience. So if you like basking on the beach all day- this isn't the place for you.
To answer your question: ditto from the 1st response- don't stay in town. there is nothing to do. We went on the Kayak/snorkling tour. It was really fantastic. Then we got back to the ship for lunch. After that we still had time and we found a guide to take us to Trafalgar Falls- just beautiful. There is a hot spring there too- just excellent fun. So if you don't want to be too physical- the Trafalgar Falls trip is great. The hike isn't demanding at all.
I totally agree do a tour. I went in January with my husband and we did a tour offered by Carnival that took us to Trafalgar Falls, The Emerald Pool and a tour of Roseau. I thought it was an interesting island because it seemed unspoiled. I will be returning in Jan on my cruise aboard the Destiny and am taking the whale watching tour. Hint if you are looking to buy cigarettes Dominica has the best prices. Go to the store towards the left after you get on the street. Happy Cruising!