Most attractions, museums, and many of the restaurants in the Black Hills [1] region offer discounts to seniors, so feel free to ask about discounts even if there are no posted signs. The state also offers a discount on senior fishing licenses if you are planning a fishing outing while in the hills. Many of the hotels in the hills offer discounts to members of AARP (American Association of Retired Persons, www.aarp.org [2]) as well.
For a $10 fee, good for life, all seniors (over age 62) should consider purchasing the senior America the Beautiful—National Parks and Federal Recreation Lands Pass. In the Black Hills, it will allow you to enter Badlands National Park [3] for free and provides a 50 percent discount on cave tours at both Wind Cave [4] and Jewel Cave [5]. Other federal sites in the Hills may offer discounts on camping or other facilities for pass holders as well.
The organization previously known as Elderhostel is now Exploritas (800/454-5768, www.exploritas.org [6]). This organization has historically offered reasonably priced educational tours to seniors over the age of 55. The organization has lifted most of its age restrictions but still provides activity-level requirements for the tours, to assist seniors in selecting a tour that will match their individual level of fitness.
Links:
[1] http://www.moon.com/destinations/mt-rushmore-the-black-hills/discover-mount-rushmore-the-black-hills
[2] http://www.aarp.org
[3] http://www.moon.com/destinations/mt-rushmore-the-black-hills/the-badlands/badlands-national-park
[4] http://www.moon.com/destinations/mt-rushmore-the-black-hills/the-southern-hills/wind-cave-national-park
[5] http://www.moon.com/destinations/mt-rushmore-the-black-hills/the-southern-hills/custer/sights/jewel-cave-national-monument
[6] http://www.exploritas.org