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.
Warm summer nights should be spent in the high grass in Hampstead Heath above the ponds. For midsummer bring herring, snaps and songs and stay until sunrise!
Nice old trees, lots of space, good for walking, running, hanging out with your kids and most of all, the perfect place to lie in the grass and chill out.
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?!