I go to the Heath every weekend (often via the wonderful Waterlow Park which skirts Highgate Cemetery) and during the week too if I’m working from home; it helps to put things in perspective when I’m thinking about my work. It’s great in the summer for obvious reasons but I love it the most in autumn and on those rare days it snows so much London grinds to a halt (which doesn’t take much). Everyone heads up there to make giant snowballs/cats/creatures and surf down Parliament Hill on anything they can get their hands on. Grown ups turn into kids for the day. My husband and I made this snow yeti last year, it was sad to leave him behind but he soon made friends and even made it into the Camden New Journal.
I love how much you are aware of the changing seasons out of the city; there are numerous beautiful forests around Hertfordshire to go for a run, a walk or have a picnic. One of the best destinations for a day out from London might be Northaw Great Wood. The main entrance is located on the Ridgeway, the B157 from Cuffley to Brookman's Park.
The many book stores dotted along this strip can provide hours of inspiration. Aside from those selling new books, several specialise in second-hand and antiquarian books.