Given that you like classic rock, I'd suggest an electric guitar, because I was int he same boat.
My wife has been watching me (annoyingly) air-guitar around the house for 12 years, and finally got me a real instrument for xmas. I'm learning to play as we speak.
I know an acoustic is better to learn on, but this is strictly a diversionary hobby for me. I also know the setup I have is total crap, but again, I don't care because it sounds awesome to my untrained ear.
I went with a "starter kit" from Costco - a "Stratocaster-like" Fender guitar, a 10-watt amp, cable, strap, bag, a few picks, and extra strings. You can literally plug it in and start playing. It was about $180. On top of that I added a $100 Digitech effects pedal that will approximate everything from U2 to Slayer.
As others have said, don't forget the free lessons that come with Garage Band... and the thousands of YouTube videos. Chances are someone has a video explaining how to play your favorite song.
It's taken me three months of maybe an hour-per-weekend to get to the point where I can play a punk-pop tune or two all the way through by memory. I still suck, yeah, but it's fun when you can see yourself actually progressing and making recognizable noises.
