I didn’t know it existed until I moved to Brixton. Brockwell Park has been a revelation to me, it’s beautiful with many secrets. Check out the outdoor theatre in the summer up by the old house - Midsummer Night’s Dream last year on a midsummer’s night was astoundingly good.
Slightly biased as this is where I live, but Stoke Newington is great place to spend lazy saturday / sunday afternoons getting sloshed with friends. There are 10 + pubs within walking distance who all are great places for one reason or another. I'll pick out the White Hart for its huge beer garden...