If your time frame is only 30 minutes prior or post exercise, I would suggest after a workout. Eating that close to working out can upset the stomach and mess up your cardio time.

Eating is something you need discipline on with regards to time as well as choices. You need to eat something 30 minutes after you wake up. Your last food should be no less than 3 hours from bedtime. Same goes for drink but not as important.

The only thing I could eat 30 minutes before a workout would be half an energy bar. If you can't eat beforehand and your are pressed for time, you might consider getting a post-workout meal/energy drink like something from EAS.