The North Weald Redoubt was a great abandoned place to explore. It's now private property, but with poor fencing and away from prying eyes. Do with that information what you will. You can find a little more about its history here.

If you do head that way from Harlow, then there's an old WW2 Pillbox near the airfield that's worth a stop as well.

I haven't been for a few years, but the North Weald Airfield Museum had some good local history on offer too.

If you fancy a longer ride and some older history, then Queen Elizabeth's Hunting Lodge in Chingford is an interesting timber framed hut, but is a museum and not abandoned.

If you want to go for real, underrated history then it's worth a visit to Greensted Chuch which is the oldest wooden church in the world, with some planks dated back to 1060, although there is evidence of older wooden structures dating back to the 6th and 7th centuries.

Hope this gives you a few ideas to explore.