If you're looking for some green space, why not try Queen's Park in NW6. It's always fun to visit new areas in London and parks are the perfect excuse. Queen's Park is one of my favourites for an afternoon or morning trip to occupy the kids.
Between the giant sand pit (stocked with slides, swings, climbing toys and the like), a paddling pool (open only in the summer months), and the mini children's farm, kids will be entertained for hours.
Until August 31, free entertainment at the pavilion in the form of magic, Punch and Judy shows, and juggling are performed several times a week. Click here for a current schedule.
Queen's Park, Free
NW6 - Zone Two
Nearest tube: Queen's Park (Bakerloo Line), 5-10 minute walk from the station
More information found here.