Walking, biking, or Bar Method (DVD.) I'd like to take up kayaking one of these days though... As long as I'm outside and going somewhere, I'm happy. My dog keeps me going too, no matter the weather or how I'm feeling.
I haven't tried any of these newfangled cardio/dance/Zumba/spin classes due to not having a gym in rural Japan. Right now I'd say having a good long outdoor run is my favorite, nothing beats that "high." Also marathon hours-long nonstop dancing to techno.