For under $50 you can do the most intense tour of any national park at Mammoth Cave: The Wild Cave Tour. Don't let the name fool you-- it's way more intense than the word 'wild' describes...

Following-up on my article last month,
Adventure Trips Without Going Broke, I spent this past weekend exploring Mammoth Cave National Park in Kentucky.
Cave exploring, or spelunking as some call it, involves twisting and contorting your body as you walk, crawl, and struggle through narrow passages, rocky cliffs, and running water.
For less than $100 you can spend the weekend camping in the park, do the Wild Cave Tour, and make some new friends.
The tour involved exploring some of the most cramped and more recently discovered parts of Mammoth Cave; parts like the "Cheese grater" (discovered by a tour guide named Mr. Sunshine), "Tall Man's Agony", "The Cathedral", or "The Man Hole".
$48 books you a spot on the tour, but make sure you book it at least a few weeks in advance as tours usually fill-up. Also, be prepared to spend extra money if you don't have a couple pieces of important gear, namely knee pads and ankle high hiking boots (both of which are VERY important).
You can expect to spend 6 hours exploring 5 1/2 miles of the cave up to 320 feet below ground so this tour isn't for those who don't want adventure. Beware there are some physical size limitations for participants and they are for good reason as those of you with claustrophobia would agree the tour goes through many very tight spaces. I'm a bit claustrophobic myself so the tour was both a physical and mental challenge, but I made it through and it was awesome.

While you're visiting try camping in the park's main campsite.
For about $24 per night you can pitch your tent and have access to bathrooms, a fire pit, and a place to park your car. This was actually my first time camping and it only helped to enhance the experience of adventure. As one of our fellow campers discovered you can find some dead wood nearby or spend $4 at the park store for a small bundle. Showers are available for $2, but as our potent team could tell you aren't necessary with some wet-wipes and congested, cave-dust filled noses! Some of our fellow spelunkers and new friends who ended up camping with us may not agree with our foregoing the showers, but we did have to save money for a few other normal expenses like gas and food.
All-in-all, a $48 tour + $48 campsite for two nights + $4 of wood = $100. You may have some additional gear and travel expenses, but it beats the cost of flying down to the Amazon and getting malaria shots... for now, at least.