More here, and in various other posts on the forum if you do a search: Poole harbour good idea?
Essentially loads of places inside the harbour to explore in a small boat. Brownsea Island is lovely and you can go ashore, pleasant trip around the back of Furzey and Green Islands (both private) with various anchorages. Arne is a quiet nature reserve you can explore on foot, nice place to anchor behind Round and Long islands if the wind is right, or put it on the beach. A passage up the River Frome to Wareham is always good (but slow), you can berth alongside at the village quay. Poole Town Quay offers easy access to Poole itself. Or if you fancy going outside the harbour, Studland and Swanage within fairly easy reach, or Bournemouth Pier the other way.
I hope you have a great visit!