I lived in Highgate for a year, near Alexandra Palace. The whole area is residential on the outskirts of London ('Zone 3' but just 20 minutes from the center) that is second to none and is very nice. It sounds corny to say this but take a walk around Alexandra Palace (it's often used for events) and marvel at the views from there that are delightful.
Obviously when you visit a city like London or its surroundings, spending a day at an amusement park is not among your main ideas. But if you have time, go with friends or fancy a day away from monuments and subway stations, Thorpe Park is a great option. The park always has a great atmosphere but its not very large as it's on an island in the middle of a lake. But it has many very good attractions (some are the best in Europe) the decor isn't very striking, each attraction has its own, but overall it's very good, with lots of greenery and there are some very nice lake views from the park. The entrance fee is a bit expensive but if you buy from the website it's greatly reduced. It's not a good idea to go with young children as most of the attractions are rather powerful.