You could swing by the bandstand at Kensington Gardens to do a little Scottish country dancing. It's run by The Royal Scottish Country Dancing Society and it's on from 2 till 4.
Or if being swung around by men in kilts isn't your thing then take a picnic to the bandstand in St. James Park and simply watch and listen to The Watford Brass Band. 3-4.20 or 5-6.15pm. Looking at these pictures it look pretty quiet and you might be lucky enough to lounge on the deck chairs.