I would definitely suggest Fat Tire Amber Ale as a good starting point. It's a bit darker than light beers, but it's still very easy drinking. George Killian's Irish Red is a darker lager that is also pretty easy drinking. You could also try some wheat beers; most light beer drinkers seem to like them.
After that, you could try Newcastle Brown or Sierra Nevada Brown. The SN is actually one of the best brown ales I've ever had. As for really dark stuff, try Anchor Porter or a doppelbock like Paulaner Salvator, Spaten Optimator, or Ayinger Celebrator. All great beers.
If you want to try hoppier beers, I think Full Sail Pale Ale is pretty decent, and when you're ready to move on to India Pale Ales, I'd recommend Pyramid Thunderhead IPA and finally, the big daddy, Dogfish Head 90 Minute Imperial IPA.