There is research (K-State U 1985, International Journal of Sports Medicine 1998) that shows you should do your cardio in the morning, on an empty stomach in order to maximize fat burning. So, in that case, you'd eat nothing. After that, you would eat a nice, balanced breakfast (with no white flour, refined sugar, or fructose corn sweetener) According to the research, this is only beneficial if you do short, intense cardio workouts (the kind that seem to work best for me and many others). Go longer than 30 min and you may start burning lean tissue for energy because you have no stored glycogen.
There is research on the other side that says that you should have a small amount of fruit and some protein supplementation in order to maximize calorie burn and recovery capability.