If you haven't yet explored the Lee Valley Park, one way to get an idea of what's there is from photos on Flickr. A good starting point is a Flickr group called The River Lee and Lee Valley. Anyone can view the photos - you don't have to join. The weblink is here. (Click now. You may be surprised at how attractive it can be.)
There are now one hundred members who've posted photos from along the Park's 26 mile length. Many photos are geotagged so you can locate the spot on a map and plan your own trip.
There's easy access to the River Lee towpath from near Tottenham Hale Station. If you take photos, consider joining Flickr and adding to The River Lee and Lee Valley group. It's another way to encourage more people to use and enjoy the Park. And the more people use and value the Park and River, the stronger the resistance to those threats it faces. Like any attractive waterside or green space in London, the Park will always be eyed by developers, and other people who want to slice bits off; or build a few towerblocks to sell the view.
As the daughter of a very knowledgeable bird watcher (and a bit of a bird watcher myself) I am often to be found chasing small brown birds across muddy ground and we have found the area around Waltham Abbey (easily accesible by train) as well as the walk up from Enfield Lock to be very good. We enjoy taking the kids round Walthamstow marshes although as it is busy, especially in summer, not so great for the binocular brigade. We are v lucky to have such a lot of wetland and river landscape so close and I absolutely support Alan's comment that it needs to be used and supported as it would be a disaster for wildlife if the developers are allowed anywhere near it.