Daytrips from London

simple - portsmouth. 1hr 20 mins. perfect.

Whitstable.

Winchester.

If you want to do hiking in the countryside - then maybe a city is not your best bet.
I know you've been to Oxford - but try Charlbury or Woodstock (Hanborough Station)- both a few stations on from Oxford.
You may have to change in Oxford - there are some through services from Paddington

At Woodstock you can do a glorious 7 mile walk - centred around Blenheim palace - and at Charlbury a very pretty 7 mile walk around the Wychwood Forest

Both examples of rural Cotswolds at their best - when it isn't raining.

David

Hever castle - nice old house, glorious grounds, interesting history
Portsmouth to see the Victory and dockyards is well worth a day out (town itself is crap)
Blenheim Palace is another great day out. Superb palace, fascinating tour and the superb grounds
Hampton Court is within London but a great day out and easily reached by train. Superb palace, nice grounds and interesting maze.
What about Winne-the-Pooh's home at Ashdown Forest?
Windsor castle and surrounds (eton) are great for a day trip.
Train and bus to Sissinghurst

New Forest

Dungeness in Kent. Bleakly beautiful coast, a national nature reserve plus a private estate that you can enter, really strange place in general. People living in a 'village' of shacks/houses mostly created from old railway carriages (!), Derek Jarman's garden, a lighthouse, plus you'll have to get there by riding on the world's smallest public railway! Bit more of an effort to get to than some of the other places suggested but worth it. There's also an art gallery and a pub. No crowds. wiki on Dungeness

Bristol (rather than Bath as first choice, but Bath also).

Also Cheltenham.

Warwick.

If you don'twant to pay to go into the castle just hire a rowing boat and see it from the river. Visit the big church (climb to the top) and wander around the old streets.

I think it is about 90 minutes from Marleybone.

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