Not the most exciting place inside, but a lovely rooftop area which not all that many people seem to know about, 2 mins walk from Old St station. If it’s packed then The Red Lion is just up the road.
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...
A great place to start the day if you are in South London is Federation Coffee in Brixton Village. The food is great and so is the coffee. This is a very chilled place where you can get both and indoor and outdoor experience (even though the whole place is under the Village roof).
I am definitely not precious about the materials I use to make my work, but every so often I find myself in need of something more specialised. L.Cornelissen & Son is a beautiful and well stocked art supply shop with extremely knowledgable staff.
Camping in London? Absolutely! This is one of a few sites which has direct access to Epping Forest from the campsite. Easy to get to by tube, with a fire field and 50 acres to spread out in, this is a great place to recharge your batteries.