I love London, but after a few days I always appreciate escaping the city to be surrounded by nature. Seven Sisters Country Park, situated in South Downs National Park, is a perfect day trip. Catch the the train to Seaford, which will take about 90 minutes. Before you begin your journey, stop at the grocery store in town to pack a picnic lunch.
You can walk the seven chalk hills for which the park is named all the way to the next town, Eastbourne, where you can catch the train back to London. The walk is about 12 miles and takes you past gorgeous coastline, seaside cottages, and two lighthouses. You'll feel a world away from the big city.