Does the event provide free water refills? Ask! Bring what you need so you don't have to buy disposables on-site, but balance that against packing light to keep your vehicle's fuel efficiency higher. Simple living is doubly beneficial when you're traveling!
Before you walk out the door:
Shutting all the lights isn't enough if you'll be gone for more than the day, unplug all appliances at home before you leave. Older TVs, many appliance chargers and other devices keep running even when they're "off."