Hey!
I will most likely be going to London this spring or summer for 4-5 days. I will stay near Tooting Broadway (along the Northern Line in southern London).
I’ve been a few times, and since I’ve ticked off most touristy spots, I’m now looking to escape the city for day trips.
I’ve been to Brighton, and as much as I like it, I’d love to go somewhere small, a cozy town or village with lots of greenery that is very walkable and easily accessible by train.
Any recommendations? 
I used to live an hour from London by train.
So many interesting villages around where we used to live, but not all are so easy to get to by train.
A lot will depend on the weather, when you’re there; as is always the case in England.
It could be shorts weather, or jacket and umbrella.
There really are so many options, one could write a book on the subject: ‘Day Trips From London’.
Maybe that’s my calling. 
How far do you want to travel?
Under an hour, two hours?
What’s more important?
Do you want to go hiking (long walks), or are cosy cafés and pubs more up your street; with a view of the village green, of course.
Countryside, villages and nature escapes; quaint towns and picturesque villages; or coastal vibes but still relaxed?
Some quick thoughts: Richmond (and Kew Gardens); Box Hill (Surrey Hills); Marlow (Buckinghamshire); Old Amersham (Chilterns village); Henley-on-Thames (Oxfordshire); Lewes (East Sussex); Saffron Walden (Essex); Dedham (Constable Country); Whitstable (Kent).
Have you been to Windsor?
How about Greenwich; Hampton Court; Hever Castle; Dover Castle.
You might want to look at what Visit London have put together - 16 best day trips from London.
Now to make a draft outline for the book
@TravelNotes , thanks for these great suggestions!
So up to like 3 hours max would be ok.
Honestly, I would want to do all you just mentioned: long walks, village greens, quaint towns.
So the list of your recommendations scratches a lot of those natural and cultural itches.
A big part of my adventuring centres around music, books, movies, and sports. So as an example: Whitstable in Kent would be a great one as I’ve been meaning to go to Margate (I am a big fan of the band The Libertines - they have a hotel, bar and studio there).
I’ve been meaning to go to Richmond (as a Ted Lasso fan), so good reminder there.
What is your personal favorite day trip from London?
The next one.