On a hot day there is nothing better to do in London then go for a swim in the Heath ponds. You get completely transported you out of the city as Hampstead feels like being in the countryside except you're in central London?!
The Design Museum is a museum in Kensington, next to Holland Park, which covers product, industrial, graphic, fashion and architectural design. There is also a nice cafe on the 1st floor with beautiful views of the park.
A small, independent cinema with a lot to offer the Brixton wanderer. As well as operating like you would expect a converted theatre-to-cinema to, Ritzy offers exclusive, niche film viewings, two bars and an upstairs live music venue. All of which provide an amazing platform to be 'culture-slapped' into shape.
I grew up in Brighton which is full of cheesy bars like this. If you can handle being surrounded by multiple screens playing Kylie at full volume, its pretty much the cheapest bar in central london. The cliental is so varied its great for people watching and completely un pretentious.