I've done a few hackathons in my time, and what I've come to realize is the best part about hackathons is being free from the usual constraints of making a highly polished robust project.
Now, I want to start off with that while "Hacking" does have a technical meaning, in this context it's more used for "cobbling" something together to make a prototype/proof of concept/MVP(Minimum viable project). There are differences in the level of complexity as it increases in the same order they're listed in but the general concept is to just make something and get feedback on it.
The best thing I've found when doing hackathons under high time constraints (<1 week) is to just start stubbing out pages for everything you'd want to have on it, and then start at the start of the user flow and start fleshing them out.