I bet you could play Half-Life or CS 1.6. They're $10 each, and CS 1.6 is still an awesome and very addictive game. I have Source and I still play 1.6 some.
Also try Quake 1, or 2 (not 4 lol [but maybe even 3]), or Unreal Tournament (not 2004 lol)
I recommend CS 1.6 the most of all those. Although I haven't played the old UT.
Steam: TriplePlay2425 (Nickname: |FCLH| Pumpmaster Flex or |FCLH| Col. Toby Mustard IV)