Harbour Island is 3.5 miles long by 1.5 miles wide, a small, quaint island of narrow streets filled with pastel cottages surrounded by palm trees and lush bougainvillea. I don't mean you won't see dilapidated houses and piles of trash here and there, because you will. Nonetheless, we were delighted by the old homes and the down home island flavor of everything. They have an enormous chicken population on the island which actually may surpass their local population. The hens and roosters pop up everywhere as you walk or drive around; the crowing goes on all day but you get used to it. The beach is the charm here; it is 3 miles long and a heavenly pink sand against the turquoise water. If you like to run or walk on the beach, you will love it as the sand is firm and easy to exercise on. 
The main mode of travel, aside from walking, is by golf cart; we rented one for the week. We got ours through our hotel (because we didn't know when we arrived it would be a necessity) although someone later told us the cheapest place to rent was right near the dock. Most activities are water or beach oriented and you have to be able to find your own amusements. If you want to be entertained or are looking for activities for your kids, this is not the island for you. It's very laid back, you sure won't need a tie or a little black dress and if you want nightlife, you mix with the islanders at their haunts. Surprisingly, you can ride a horse bareback on the pink sands of the beach- they were located right next to our hotel on the beach. We found tooling around in the golf cart a really fun way to see every nook and cranny of the island, although we got soaking wet the one day we spent sightseeing. Tropical Storm (soon to be Hurricane) Gustav was stirring in the Caribbean during our trip and we had quite a bit of rain.
My daughter and son are divers so they had looked into diving groups on the island. There appear to be two and Valentines was recommended by a friend; unfortunately, because of the weather they were only able to go out three days, and although they had one frightening day with some inexperienced divers, they were pleased enough with the dive group; the dives were good, but not outstanding. If you like to shop, or just want to wander around, you will find lots to do in Dunmore Town; we liked shopping in the afternoon and found straw vendors lining Bay Street, nice jewelry, clothes- lots of decorative odds and ends along with the t-shirt shops. Look especially for Dilly Dally, Island Treasures, Shells and Things and The Blue Rooster. You'll find lots more just walking around. Unfortunately, the art gallery was closed. Our biggest help locating the interesting shops was our breakfast waitress; she was nice enough to name the best and locate them on an excellent map of the island. The Pink Sands Hotel has a nice boutique.
One of the quirkiest sights is an intersection referred to as "Uncle Ralph's Aura Corner". It is a collection of hand painted signs, some printable, some not. I read someplace that if you take a photo, you should leave some change and Uncle Ralph will give it to the medical center. Unfortunately, we did not know that at the time.
A friend recommended our hotel, The Coral Sands. It has a sister hotel, The Pink Sands, which is double the price yet located right next door and they are on the identical stretch of beach. We were very happy with the hotel; our room was not luxurious, but excellent and was air-conditioned which was nice as it is almost a necessity what with the humidity of August in the tropics; however, our bathroom left a lot to be desired and I believe it was scheduled to be remodeled. The rooms are no doubt the difference between the two hotels, though we did not take the time to look at a room in The Pink Sands. Coral Sands photos below (lobby, pool and dining area):
The lobby of The Pink Sands:
As far as eating was concerned, we were rather unlucky as many places were closed due to the time of year so our choices were limited. Food is expensive here. The one grocery story has very little in the way of food and after speaking with someone who has a home in these islands, they have to go quite a way for decent provisions. As on most islands, food has to be imported and this always makes for pricey meals. If you go on a budget please keep this in mind; our meals were costlier than we expected. However I must tell you the chefs on this island are superb. We never had a bad meal anyplace. I would not be exaggerating to say our meals were spectacular. We are foodies and therefore fussy and knowledgeable about dining. We ate at our hotel, The Coral Sands, at The Pink Sands and a restaurant in a hotel called Rock House Hotel, which looks like a fun place to stay (the sunsets there are unreal) but unfortunately it is not on the beach which we wanted. Be sure to order a Goombay Smash here! Sunset at Rock House and their pool area below:
Of course there are lots of small, inexpensive places to eat (and takeout) as well. You will find many of them scattered around Dunmore Town; we especially loved Queen Conch. They have odd hours though (that could have been the timing of our visit- off season); you place your order and tell them when you are coming back to pick it up. Their conch salad is to die for. They actually have listed possible ingredients and you can pick and choose what you would like in it. 
Ask your hotel or any locals where to go for lunch and dinner. Or check out this website which I think is informative and amusing: http://www.myharbourisland.com/As far as nightlife is concerned, try Gusty's or Vic-Hums or both!
The old fishing dock, where you can buy fish:
We noticed a big influx of people on a Friday and decided that many Floridians come over for long weekends; it is such a quick trip from Florida and the time it takes to get from the airport to the dock to Harbour Island is under half an hour. The one thing you have to be ready for- flight delays. Most of us are already used to delays, but there are lots more in the tropics because of the frequent (and quickly formed) storms that arise in the afternoons.If you are looking for a relaxing time on an cozy, laid back island, you will love Harbour Island.
