For a city experience completely unlike anything you might find in London, head off to Colchester. The oldest city in all of Britain, Colchester was the first settlement of the Romans, so many centuries ago, and has some astonishing history behind it. It's around 50 miles outside of London and is home to its own castle, as well as some interesting museums and historic sites.
Getting to Colchester from London is easy enough on the train. You'll need to get to Liverpool Street Station via the London Underground or buses first and then take a Greater Anglia train direct to Colchester. The journey takes around 45 minutes, and if you visit Colchester on a sunny day, you can extend your trip over to nearby Mersea Island for some super seafood restaurants and stunning sea views.