Invite small companies that sell open pollinated seeds, not hybrids. Many people will not have seeds to swap the first year so will need to buy seeds. You will be amazed at the amount they had made during the day. Decide if you want to have an admission charge or a simple donation box at the door. Make a sign telling people what the money will go towards.
Donations go to various conservation projects.
The setup committee can decide what to donate money to. Money could also be used to set up a community heritage garden project or support a community garden project. Think about becoming informed about seed saving. Seeds of Diversity Canada. The Heirloom Gardener.