I know when I was taking Bonnie to dog shows some of the people would give their dogs this high energy gel stuff for dogs to get them energetic and perky. I have seen people in agility doing the same sort of thing, kind of a nutritional boost:
High Energy Nutritional Supplements
Giving her a small bowl of food when you get there is a great idea. You could just give her a can of meat. If she is working hard, she does need extra nutrition for the workout.
The freezedried raw food is great, too. I give it to my dog as a treat, but they do need access to water if you give a lot of it.