Tate Modern can always be relied on for offering up interesting, challenging exhibits, housed in great architecture and with one of Londons best cultural bookstores. An ever reliable weekend hangout.
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...