Hi there - I know Sunday has passed, but it'll be here before we know it again!!
On the countryside walk front, there are some lovely walks in the Surrey Hills - so only about 30mins drive depending where a bouts you live in the area - Box Hill for example. There's also somewhere called The Lookout - which is also great for toddlers when the weather dry.
http://www.bracknell-forest.gov.uk/thel ... verycentre - loads of walks there, a Go Ape and lots more....If the weathers bad there's a discovery centre there that keeps them entertain!
Might also be worth joining the national trust - about £80 for the year, but a lot of the local national trust attractions have lovely walks - Claremont Gardens for example.
Other options are Hampton Court (the maze keeps the kids entertained for hours), Richmond Park, Bushy Park, Marble Hill, Syon House (there's also a soft play there is the weather's rubbish).
Also if the weather is rubbish there's a soft play in Leatherhead, Kingston...
The local swimming baths are also a winner if it's pouring outside! Hampton Outdoor Pool is heated all year around!