Food and beverages! Know your audience; vegetarians, vegans, halal and so on – make sure you ask your attendees before hand. You should also have a food plan over when the different foods will be served. And as a last tip, I would not recommend serving alcohol, however people can bring their own if they want.
CASES! CASES! CASES! Create 3-5 cases that people can choose, if not, at least be inspired. These cases should be short and easy to understand.
Example case from our hackathon: Create a playlist based on your surroundings such as weather, time of day, location and so on.
API! API! API! Find a catalogue/platform of APIs your attendees can choose from. Typical APIs can be resources such as weather, time, language or other location-based information.