Black Hills off the beaten path

We are going to the Black Hills this summer and will be there for 6 days. We will spend the first 3 or 4 days doing the usual "tourist" places, but what we are really looking for is the "off the beaten path" areas. We love to hike, so we are looking for ghost towns and camps, waterfalls, creeks where one can do a little panning and rock hunting and things of that nature. We are in our 50's so it can't be too awful strenuous, but we would really like to spend some time away from the crowded areas. I would love to hear from some folks who have been there or who live in the area and know where some of these "hidden gems" are.