I love swimming and I love being outside. London is great if your into both these things. I swim at London Fields Lido which is heated all year round. Its best in the winter when the steam rises from the water and if your lucky its snowing a little on your head. If your brave you can also go for the unheated option of the Serpentine or Hampstead Ponds.
Small restaurant on Highbury Corner that started off focusing on (basically) posh kebabs, but which is now producing some of London's most innovative dishes, focused around unconventional meats, cuts, and breads. Not often you can eat incredible food while listening to heavy rock and metal, but Lee Tiernan has nailed a winning formula.
These giant spectral cylinders are great at different times of the day but my favourite time is around dusk when the buildings in the city start to light up. The Gherkin and the new Shard building are framed in the lowest ring then the shifting colours of the sky are almost split in the following ellipses.
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).
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.