Is there anything fun to do in Korea that's not too expensive? The USO trips are way too expensive!

It depends on where you are going. Take the buses between bases. They are not bad on prices.
Go to the Buddhist Temples and Fortresses.
Visit the military museum by the Korean War Memorial...you can spend a whole day there.

We would take a bus from Osan to Yongsan and walk to the train or down to Itaewon. It does take a little getting used to, but it's not at all intimidating.

During my two 1 year tours in South Korea, I enjoyed several weekend trips to Seoul. The Dragon Hill Lodge at Yongsan Garrison is easily one of the best Armed Forces Recreation Center and Hotels in the world. Seoul is a huge city with everything you could want from shopping, site seeing, great museums, great parks and walking zones, great restaurants and great bars. If you can't find something to do in Seoul you're basically...hopeless. I will say that if you have a fear of crowds, then Seoul and its 10 million plus population, is not for you. Seoul also has 3 amusement parks, which I'm told are fun. I thought the DMZ tour was bizarre but worth taking. I also attended two of the banquets held by the South Korean government for US Korean War veterans. As a country they went all out to show thier appreciation.

I stayed at a Buddhist temple overnight. It was a once in a lifetime experience that I would not do again, but I enjoyed it.