Who are you calling cheap?
I must preface this by saying that, as a no-child household, we are already ahead of the game. I don't even want to think about how expensive a sitter is these days. I remember my babysitting days, and they were pretty lucrative.
Anyway. My favorite cheap date with consists of two bottles of water each, some Gorp (or a Cliff bar), and our annual pass to Red Rock Canyon. The park has so many trails, from an easy stroll into Ice Box Canyon to the more challenging scramble and climb to Turtlehead peak. We throw everything into a knapsack and head off.
Check out state and national parks in your area, as our annual pass was only $20.00. Very often there are plenty of other programs to enjoy-- group hikes, star-gazing, children's hikes.
Head on over to Shalee's Diner and check out everybody's ideas.