We camped at Buddens for a bank holiday weekend camp, visiting Brownsea Island, mountain biking in the New Forest and visiting the beach at Bournemouth. We also went to Brenscombe activity centre, but I wasn't overly impressed, and wished we'd gone for the activities at Buddens instead. We got a good deal with National Trust membership, with access to Brownsea, and entrance to Corfe Castle on the Monday morning.

<a href="http://www.buddens.org.uk">Buddens</a>

I'd recommend Buddens as a base for visiting Brownsea.