The NH Seacoast has a number of beaches for you to try out even in September if the weather is nice a day at the beach can be fun.
Portsmouth has a number of Museums for you to check out including a Childrens Museum.
Out Door Parks
The Portsmouth area has a number of parks to choose from from Prescott Park in Downtown Portsmouth right on the water front to Fort Foster just over the bridge in Kittery ME.
There is plenty of shopping that you can do in the Portsmouth area from the local shops downtown to the Kittery Outlets in Kittery ME.
For a full list of things to do in the Portsmouth area please check out Portsmouthnh.com