No prize is definitely the way to go. My university hosted a 24 hour, in person hackathon every spring. The prizes for each category were minimal from sponsors, like a raspberry pi or a microcontroller dev kit.

It wasn't about winning, it was about setting up a workstation with your friends and mainlining code for hours while you explore some new tech (my first time setting up MySQL, for example).

Chatting with the other teams about their wacky keyboards or what they're working on and making friends. Lots of good times.