A short but varied circuit on the rolling hills of the Chilterns, this walk has a nice variety of grassy ridges, archaeology, and wide views out over the Vale of Aylesbury. There's plenty of nature too: the Ivinghoe Hills are an SSSI, with areas of unimproved chalk downs, semi-natural woods and scrub, and wildlife including rare butterflies, kestrels, and red kites. Close to a train line, and with quick road access from London and surrounding towns, it's an accessible and scenic half day out for hill-starved southerners, and could easily be extended into the woods to the east to make a longer loop.
I used to live pretty close too, used to be a regular walk, or mountain bike ride if you start early. Avoid in the rain though, the mud becomes horrible sticky grey gloop. There is also a good much longer walk that goes around the Ashridge Estate boundary.