Def make a day of it. Plenty of space to play and picnic, red squirrels to find and sea to paddle in. Scout and Guide trading post very interesting if you are that way inclined. Interesting small church and various NT interpretation. If the weather is good its a magical day out. Ferry over/way back often does a little commentary tour round the harbour. Might not be relevant but if you have young children/toddler you can hire a Mountain Buggy pushchair from the NT - we did this with DS2 a couple of years ago.
Gosh you've just made me very homesick for Poole..........!!
thanks for the recommendations. we went and had a blast. spent a lot of time on South Beach, wouldn't have thought to take swimming costumes without mumsnet. I can imagine living at Poole brownsea really must have been a favourite visit!