Regent's Park in autumn is a wonderful resource. You can play softball, run, take a horse on its sandy tracks (but you're not allowed to gallop - a canter is as fast as you go), or play tennis or just laze in the sun.
But until recently I didn't know about the Hub. Now, this list of activities has expanded to include Pilates, Yoga, Power Walking, Tai Chi and Power Stretch. There's even mother and baby yoga.
And this comes on a 'Pay as you go' basis - no need to pay gym fees.
The Hub is quite an intriguing building - it looks like a flying saucer partly buried in the earth. The top floor is a cafe with extensive views of the park and a roof apparently modelled on the spokes of a bicycle wheel - below are practice rooms and changing rooms. It's only a couple of years old, and it's definitely a step up from the usual smelly changing rooms associated with playing football in the park. (And the good news is you can use the changing rooms, for 1.50, even if you're just playing five-a-side in the park and not coming to one of the exercise classes.)
There are classes for children as well as adults. More info is on the Royal Parks website. The price of classes is very reasonable, at between 2.50 and 6, with special rates for the unemployed and over 60's.
And if you're getting there via Baker Street tube, you get the added benefit of a 15-minute walk each way. Definitely enough to get your heart beating that little bit faster.