Check this out! Military get FREE passes to the national parks and wildlife preserves. Just go here --> http://www.nps.gov/findapark/passes.htm and sign up for your pass! Mammoth Caves is just one place that will give you free admission with your pass!
From the site: "A pass is your ticket to more than 2,000 federal recreation sites. Each pass covers entrance fees at national parks and national wildlife refuges as well as standard amenity fees at national forests and grasslands, and at lands managed by the Bureau of Land Management and Bureau of Reclamation. A pass covers entrance and standard amenity fees for a driver and all passengers in a personal vehicle at per vehicle fee areas (or up to four adults at sites that charge per person). Children age 15 or under are admitted free."