If the weather is good, less than 5m from Corfe is Studland bay - a glorious white sand beach, great for a long walk, a day chilling, and amazingly - it's utterly free of the crowds from Bournemouth across the bay*. park at Middle Beach (national trust members park free) The bay whole faces east, which is nice - as a reuslt the clear water in the bay is generally flat calm, the sun behind you / easy to find dappled shade to enjoy a day from. there are a couple of decent cafes along it, too.
Just to the south, plenty of empty, interesting/empty/stony beaches around Kimmeridge. Even the odd 'nodding donkey' - yes its one of UK's oldest still in use oilfields, running over 60yrs.
An Ancient Swannery, at Abbotsbury.
Gorgeous scenery, empty byways to drive through and enjoy the scenery, a great view above all from the Hardy Monument ... Lots and lots of good by-the-way stuff to discover for yourself. Take the B-roads; enjoy the journey.
(and yes - the Tank Museum is excellent; don't rush it.)
PS on fish & chips - Lulworth not good. The 'Harbour Lights' stand right nr the seafront at West Bay is the place to go.
(* Though you can take the chain ferry across if you must see how awful Sandbanks etc really is.)