Rainham Marshes with my wife and younger daughter. In the shadow of the A13, the C2C railway line and pylons, we completed a two-mile circular walk around the duckboard paths. It's hard to believe this area is so close to London and we saw huge marsh frogs, dragonflies, kestrels, coots and an egret all amid lots of streams and giant bullrushes.
The marshes were only preserved because the land was once used an army firing range and it's fascinating to see the old shooting butts and targets still there. The Marshes have a number of very plush wooden hides and also a giant manmade anthill that humans can crawl through — some of the real anthills on the Marshes are a century old.
The main building is very distinctive and has huge glass windows offering a marshy panorama with coffee and cakes. The walk from Purfleet station is good too, offering fine views of the Thames and an old magazine (not published by IPC) that once housed enough gunpowder to blow up a sizeable chinch of Essex. Well worth a visit.