Secure electrical lines with duct tape. It is hard to imagine that having fun on Astronomy Day can land you in court. But we do live in a litigious society, and small, unincorporated clubs can put their members at risk. If one of your Astronomy Day sponsors already carries liability insurance, they might be willing to include your event under their umbrella. Shopping malls or zoos might have special insurance arrangements for events taking place on their property.
Some malls require groups using their facilities to carry their own liability coverage.
One million dollars of coverage is typical. The Astronomical League, a large federation of astronomy clubs in the United States, offers an insurance policy for its member clubs. If your club is not a member of the League, it might be worth it to join just to have access to reasonable liability insurance. Your club could consider incorporating itself. This way its members will no longer be individually liable for guests who trip over your exhibit table.