I love interval training. Whether it's a highly-structured protocol (maybe Tabata) or more of a Fartlek approach, it's a blast. The approach I find to be both a lot of fun, and very productive, is to do a sort of Fartlek approach (with, say, running, or working on the elliptical trainer) where I put my ogg player on "shuffle" and let the song dictate my intensity. Certain songs ("Kickstart my Heart" by Motley Crue for example) just demand extreme exertion... other times you get something a little more chill ("Lights" by Journey, maybe) where you just cruise for a while.