What you eat and when will affect your physical performance along with your cognitive performance.
Depending on when your activity level is, the intensity level and length of the activity dictates how much you should eat or drink.

Here are some general rules of thumb.
Start hydrated and fueled for your activity.  Drink approximately 16-24 ounces of water 1 1/2 - 2 hours before your event.  Drink another 8 ounces 15 min before your event.  Eat a high carb snack 30 min before (avoid high fiber high fat to avoid GI issues) such as 1/2 a banana or a few crackers with a drizzle of honey.  Ideally you have had time for a full meal (complex carb, lean protein, healthy fat) 2 - 4 hours before your activity.

During exercise - depending on your intensity level and length of your activity and the amount you sweat dictates how much you should take in during exercise.  Usually exercise under an hour you will be fine with hydrating with water.  Over an hour you might need to supplement with diluted sport drinks, or gels or 1/2 energy bars or 1/2 banana.

After exercise - within 15 minutes - 1 hour eat something.  Carb/protein source.  3:1 carb/protein approx.  Replenish those muscles with the fuel to rebuild instead of breaking down.  Hydrate approximately 16-24 ounces for every pound of sweat lost.  Hydrate until your urine is clear or pale yellow.

What do you eat or drink before, during or after exercise?  I would love to hear.