Queen Mary's Rose Garden
It's right in the middle of London, you don't have to pay to get in and it's one of the best rose collections in the country. There's more than just roses, too - there's a rock garden and fountains and waterfalls. A lovely oasis in the centre of the city.
Where Regent's Park, London NW1 4NR (www.royalparks.org.uk/parks/ regents_park)