Flying between DEN and a southern city (ABQ, IAH, DFW, SAT, etc.) you fly right over Colorado Springs and Pikes Peak... quite a nice view on a clear day! (I had such a great view of the peak, but by the time I had pulled the camera out, clouds had gotten in the way...)

Obviously, sit on the "west" (D,E,F) side of the aircraft when heading south and v.v. heading north.

Also, pretty decent views DEN-So. Cal. Flying DEN-SAN will get you a pretty decent view of the Grand Canyon, for example.