Although my skating days are now over, Stockwell Skatepark located just behind Brixton Academy is one of the reasons I moved to Brixton. It’s an excellent place, lumpy and hectic but totally friendly with decent Portuguese coffee and cakes just next door.
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.
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...