I've loved the Lee Valley Park for many years now, and the area I've spent the most time in has been the section next to Cheshunt Station:
The Park covers 1,000 acres on either side of the River Lee between Waltham Abbey (Essex) and Broxbourne (Hertfordshire). It’s a unique patchwork of lakes, waterways, green open spaces and countryside area.
http://www.eppingforestdc.gov.uk/leisure_and_culture/Lee_Valley.asp
Recently I've discovered the Rye Meads Nature Reserve, an RSPB site:
Rye Mead is idyllically situated next to the River Lee on the outskirts of Hoddesdon. It forms part of the Lee Valley complex, which has a number of nature reserves and country parks, stretching from North London to Rye Meads.
You can choose from a number of paths and trails that take you round the various wildlife areas, including reedbeds, open water, woodland and meadow.
http://www.hertsdirect.org/comdirectory/comvol/enviro2y/envnaturereserve/781204
Another recent discovery is Amwell Quarry, an old gravel pit now managed for wildlife:
http://www.birdsofbritain.co.uk/reserves/herts.asp
It's a beautiful place, fizzing with birds and the county's top wetland bird watching site. I live in Hatfield, which means Amwell is just 13 miles down the road. I kick myself that I didn't find this wonderful site years ago!
I'm hoping to make Grafham Water another of my regular sites. It's a very special place, but it's in Cambridgeshire, 44 miles up the A1 so not a place you can pop in to:
Grafham Water is one of the prime bird watching sites in the county, with rare and scarce birds such as osprey and the occasional Slavonian grebe alongside the more familiar resident mallards and greylag geese. With nine miles of shoreline, and around 170 species of bird recorded each year, there is always something to see – in autumn the drawdown zone created by water levels dropping attracts a variety of waders such as common sandpiper and dunlin. Flocks of wintering and summer breeding wetland birds are what make Grafham special, earning its SSSI status with nationally important numbers of coot, tufted duck and great crested grebe. Other winter visitors include pochard, goosander, goldeneye and wigeon.
http://www.wildlifebcnp.org/reserves/reserve.php?reserveid=49
After birding in the wilds of Poland and Spain, I used to think I was "stuck" in Hertfordshire without any real access to wildlife, apart from Lee Valley. Now I'm so happy to discover that there are so many beautiful places within easy travelling distance. The next step is to explore the East Coast around Essex.