Old trails are great. Sometimes you can start from a topo map and figure out that something is there. Google Earth has good satellite photography, but it isn't quite perfect. You need to read some history books first, and then you have to get on the ground and kick a few clods of dirt over.
One good trail is the Tioga Road in Yosemite. This is not the modern paved Tioga Road where we speed along in our cars. This is the old Great Sierra Wagon Road before it was paved or straightened. It started from the native American trading route across Yosemite, then became a mine road going to Bennettville. There are places where the wagon wheel grooves are still visible in the rock, especially northwest of Porcupine Creek Campground. As you walk along this route, you find old debris piles, rusted cans, and junk. By studying the stuff, you can figure out its age. When you find a miner cabin with square-cut nails, you know you are back to 1890 or earlier.
An even better route to find is John Muir's route for herding sheep through Yosemite in the late 1860's. By reading about his first summer in the Sierra, you can find obvious places and then connect the dots across the park. I found where he had his flock cross Yosemite Creek.