How simple do you want? Most of my games are simple and ASCII, and if you want a challenge no one has yet to beat Toby the Trapper or Unstoppable (though both are in theory beatable if you make no mistakes).
Smart Kobold by Jeff Lait is a great little game that is challenging but involves little messing about.
Hydra Slayer is... well, I don't know if simple is the word, but it's *elegant*, and has plenty of interesting challenges.