The Island of Topsail is divided into three sections: North Topsail in the North and closest to Sneads Ferry and Jacksonville NC, Surf City in the middle where most of the restaurants and shopping and crowds are and Topsail Beach in the south closest to Wilmington NC as the crow flies. (You are closer to Wilmington if you stay in the Surf City area, as to leave the island from Topsail Beach, you must drive back up to Surf City.) So now knowing this information it may help you with the location of the rental, and therefor knowing what is in the area.
Recently there has been a lot of development off island near Surf City. There is a putt putt, and I believe a movie theater, and probably more by now. All three towns offer town parks, and often have fun activities for families during the season. There is a bike path on the island, North Topsail is probably best for kids, with less traffic, but sadly the bike paths on the island do not all join together.
Things to do beside beach stuff is limited, this is a non commercialized beach area where families who want to play in the surf and sand vacation. South to Wilmington, probably an hour drive-with traffic, there are plenty of activities, an aquarium, the Battleship and Ft. Fisher. North you have Jacksonville.
Karen Beasley Turtle Hospital (http://www.seaturtlehospital.org). Located in their new building in Surf City, was not open this last summer, but hopefully will be open for visitors in 2014.
The Turtle Factory in Sneads Ferry has children's activities, based mostly on crafts and painting. A really cool shop too.
Topsail can also be really good for shelling and finding Sharks teeth
I hope this helps, please feel free to ask more, if needed.
Topsail can also be really good for shelling and finding Sharks teeth.