Minet Country Park
This country park in Hillingdon includes ponds, grassland and a variety of wild plants and flowers. There's some nice footpaths to follow around the site which includes picnic areas, meadows and hedgerows. It's also home to the Hillingdon Cycle Circuit. The purpose-built road cycling circuit is about 1 mile long and a great place for a leisurely ride. The video below shows the nice wide surfaced path with views over the park.
The Grand Union Canal passes the south side of the park so it's easy to extend your outing by picking up the towpath. If you head west you can visit Lake Farm Country Park where you will find meadows, grassland, a trim trail and a BMX track.
The long distance Hillingdon Trail also passes through the park. You can follow it north and then west to visit the pretty Yeadling Brook.