I work out with a personal trainer. Those workouts are way way way more fun than working out on my own. Making my workouts fun is definitely a big part of what I'm paying him for.
If you can find a good instructor, taking a class at the gym can be equally effective in terms of making exercise fun. The group dynamic can really help.
Doing intervals makes the time fly by for me. On good days, it causes some sort of chemical reaction in my brain. I'll feel really energetic, happy, and good about myself.
It's almost like a drug. I definitely have interval workouts where it is all I can do to drag myself onto the TM to get started but I'm practically dancing by the time I finish.
Checking out other people at the gym is another fun thing to do. What are people wearing, who's cute, who's thinner than me, who's doing an interesting exercise, who's just there to socialize--it's all interesting to observe.