London is a superb place to cycle.
There are plenty of paths and the scenery is gorgeous (unless you are a commuter cycler!).
Nothing beats exercising out in nature! No smoggy streets and noisy (and rude!) traffic needed.. just a quiet lane in a nearby common or park.
You do share your cycle lanes with pedestrians, dogs, other bikes and horses, but hey.. a FAR better traffic scenario isnt it??
One can hire bikes at various spots in the City.
Well it does help if you actually turn the pedals!
Some of London's Parks (especially Hyde Park) are so large, it is possible to cycle around inside them for a while without covering too much of the same area too many times.
There's also the Serpentine lake to ride around as you can see in this picture.
I wouldn't recommend cycling round London's roads too much though unless you have a death wish or are a little unhinged!
Equipment: A bike and something to lock it up with if you stop anywhere
Biking around London is very convenient! There are plenty of bike roads and drivers take good care of bikers in general (just as they do with everyone).
But leaving your bike behind for a longer period might be a bad idea just as it is in every big city :)
Equipment: Bike and raincoat