They have a bus system that runs about half the island. If you are boat diving and staying at sunset house, the non- divers can spend the day shopping in George Towne. The last time we went, I rented a car and it was well worth it. I think it was just over 200 US for the week including the license. The car will allow you to get to the outer parts of the island, the botanical gardens, Bodden Towne, and Rum Pointe.
one thing to consider is shore diving with sunset diver's, directly across from the turtle farm. The diving was the best we had on the island, tanks and weights were 15 US and turtle reef is a great site in 60 ft of water. The non- divers could spend the day at the turtle farm, which is a popular attraction. I think sunset house also has a popular shore diving site.
if you are a large group, dining out will be your big expense. We stayed at Sunshine Suites at the end of seven mile, but they gave you free breakfast each morning and you were able to cook meals or take your lunch to eat between dives. The hotel is not on the beach, but directly across the street with a short walk to the beach via a path. The Cayman's do not allow private beaches, so you can have access to any part on seven mile, not just the public beaches.
i have been three times, and planning on returning, but like above, I fully agree that if you are not a diver it is a one and done island.