Instead of starting from scratch, consider partnering with orgs that already do hackathons.
Preparation by the project owner is the real key to success. Be organized and have ready: credentials, api documentation, access, servers, devops. If you can have a tech champion or lead prepped in advance too. Day-of time is too valuable to spend on mundane tasks.
Have a skills matrix ready before the weekend so developers can self organize by their own technology skills.
Consider charging a minimal fee for participants to discourage people signing up and not showing.