Old maps, old photographs, old newspaper articles, etc..
Some of which can be found online (historic aerials is just one website but there are others). Visit your local library and go through their old newspaper collection. Some areas have historical societies and some of those have maps/stories that are available.
I've found that talking to older people (who lived in the area their entire lives) can lead to some very good prospects - old swimming holes, speakeasies, etc... Stuff that isn't likely to be on a map or mentioned in the local newspapers.
Find the old ball fields, the old hangouts where people congregated. The old-timers remember such things and they (generally) like talking about the past.
I grew up in the mid-west and know of all sorts of places with a lot of potential - swimming holes, long gone schools, old one-room schools, houses long gone, picnic areas, sledding areas, etc.. They were places that either still existed when I was a kid, or places that my elders spoke about. Memories get passed down - your elders will have some information that they gleaned from their elders.
Pay close attention as you drive around the area. Look for fruit trees, flowers, etc.. growing in areas not currently occupied. Look for broken pottery shards, glass, etc.. In the area I live (NJ) I look for the remnants of foundations and cellar holes. North and west of my area I'd look for rock walls and other similar signs of old occupation. In parts of the country it is still possible to see remnants of old wagon trails, and the areas where many of those wagons stopped along the way.
Look for places where people gathered and money was likely to be exchanged - old train depots, bars, dance halls, etc...
Good luck and have fun
