Doesn't sound too glamorous, but you can have a great day out jumping on a train to Gravesend, seeing the town, including the Pocahontas memorial and the huge new Sikh temple, then jumping on a ferry for a 10-minute trip across the river to Tilbury. It's then five minutes walk to Tilbury Fort, a really impressive English Heritage-run Napoleonic era fort with lots of space to explore. The back to Gravesend for a pint/bite to eat and a train home.