Of course there has to be something to entertain your guests -- but just because it's fun doesn't mean it has to be wasteful.
Choose local acts and speakers who don't need to travel very far to get to your event. Once they're there, minimize their setup to reduce energy consumption and waste -- does the hot new local band you're featuring really need a smoke machine and strobe lights to wow the crowd? Probably not.
Take it a step further by making sustainability a feature of the entertainment. Guests can participate in games and trivia related to the environment. Find a speaker who has a green-centric message to get guests thinking about their own impact on the planet. If you plan to have additional activities and excursions outside of your venue, consider activities that get people out to enjoy the local environment without harming it, like nature walks and eco-tourism activities.