Poole/Bournemouth for a day..

[Brewupmama]

We will be staying over in the summer for one night at a hotel 20 odd minutes away from Poole and Bournemouth. 2 children who like the outdoors so any ideas on lovely spots to visit.
We need it to be free/cheap so no theme parks or anything
Thanks

Take them over the chain ferry (car or on foot) and enjoy Shell Bay. Idyllic white sandy beach and the sand dunes at the back for a bit of wildlife tracking :)

OR from the same place that the ferry goes from (Sandbanks) you can catch a boat which goes round pool harbour and then over to Brown Sea Island. You can get off there and wander around the woods and beaches and try to spot a red squirrel.

Bournemouth beach and pier is nice and the walk through to the sea front through the gardens.. squirrels everywhere and generally entertainment along the way. If the weather is grim, the sealife centre is on the seafront too. I love the Chines.. Durley chine is 10mins along the seafront and as a kid used to scramble up the sides and then have fish and chips on the beach. It's a lovely area for kids
I second the Pool Harbour trip too!

Depending which side of Bmth and Poole you are staying I would recommend either Moors Valley Country park or RSPB Arne, you only pay to park at each of these but can spend lovely days out at each.

Moors Valley Country Park.
You pay for parking, no entrance fee.
2 play areas, a trail through woods with stops with things to climb on, a short min-train, a lake. Lovely. (Also has Go Ape).

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Where exactly will you be staying and what do you enjoy? 20 minutes away from Poole and Bournemouth could be a quite wide area and in the summer especially traffic can be quite congested in some areas. RSPB Arne is really nice (there is a small beach for splashing around, though I'm not sure I'd really swim there). You can take the ferry to Hengistbury Head from Christchurch, there is (was?) a land train and a nice visitor centre, the beach is lovely and there's a really nice restaurant right on the beach where you get off the ferry. Brownsea Island or Studland. Kingston Lacy is more inland, but a nice day out, especially if you have a National Trust membership. There's also a hill fort just next door. If you want a non-busy beach, going more over to Purbeck and you can go to Tyneham (which is interesting in itself), but the beach there is really lovely or to Ringstead Bay. I'm not a massive fan of the town centres or Bournemouth/Poole beaches personally, but depends what you're after.

Brown sea island or moors valley. Both fabulous places.
There is a charge for the ferry to Brownsea island, and for car parking at moors valley, but as far as I recall neither are particularly expensive