Greenwich is more or less considered London and is 30mins along the river from Tower Bridge. A nice place to visit but not really 'countryside'.
Oxford might be preferable to Cambridge, it's much closer and very similar. On a trip outside London, you will mostly be on the motorways anyway, and yes you will see countryside, but it's going to all start to look the same after 30 mins. Oxford will give you a 'quick fix' of greenery, without consuming too much trip time.
You could also consider a trip to Windsor, closer still. Then you have the Royal Castle and Windsor Great Park. This is 5,000 acres of parkland, which includes a Deer Park, and is a varied landscape of formal avenues, gardens, woodland and open grassland.
http://www.thecrownestate.co.uk/windsor/windsor-great-park/
You didn't mention if this was your first time in London, but I'd definitely consider spending a half day wandering around Hyde Park. This is really lovely on a nice day, and as you get towards the middle area, you can really lose yourself in the nature, feed the squirrels (officially, don't feed the squirrels, but a handful of monkey nuts will have them eating out of your hand, literally! it's a bit of fun). 'The Flower Walk' running for 0.5m from Kensington Palace, past the Albert Memorial to the Serpentine Gallery is also really nice (squirrel central!).
As you have such a short time, and there is so much to see and do in London, maybe staying locally is a good use of time, and you could combine Hyde Park and Greenwich.
Hyde Park is more than just 'a park', it covers 350 acres and has many cafes, memorials, fountains and statues, 2 lakes and a Palace. It's 4 miles to walk around the outer path, 1.8 miles around the inner path.
https://www.royalparks.org.uk/parks/hyde-park