On my 50th we did a walking tour. It took all day but we had a really good time. Started at the Albert Hall, then crossed into Kensington Gardens, round the serpentine via Peter Pan statue and came out into Bayswater Road. Navigated up to Regents Park via Baker Street (Sherlock Holmes territory), then across the park to the far side where we walked along the Grand Union canal towpath until we came to Campden Market. A bit more street walking upto Hampstead Heath where we climbed up Parliament Hill (great view) and then on to Kenwood House (an art gallery with donations for admission). After that we left via the car park exit, crossed the road and walked down The Bishops Avenue (millionaires row) to East Finchley station and took a train back. Good exercise, fresh air and cheap!