Went on a day trip with the wife, niece and nephew last bank holiday Monday. We got the train from London Marylebone station to Wendover station in the Chiltern Hills (around 40 minutes). And then we walked from Wendover station to the top of Coombe Hill, chilled there for a while and then back to the station. It's a nice easy walk which takes around 40 minutes each way if walking at a reasonable pace and not taking breaks. The view from the top of the hill is very nice on a clear day like what we had. If you use Ordnance Survey maps, it's the Chiltern Hills North (181) map which you want (grid reference '86 07' to '84 06').
Some pictures from the walk here.