Coming up with an idea for a hackathon can be really difficult. People often worry that their idea is too "silly" or too basic. However, the thing to keep in mind is that everyone is building a project in a limited amount of time, so your idea doesn't have to be perfect. Most hackathon projects are technically unfinished in terms of edge cases and extended functionality. A lot of people take their idea and build off on it after the hackathon.
Consider this good opportunity to try and learn new things with no risked. At hackathons you can work on any kind of technology based project. People build everything from websites, mobile apps, video games and VR/AR apps - you have a world of options!
Something that helps us come up with an idea is narrowing our options down to a broader category. If the hackathon you are at has a theme or category, you can use that to narrow down what your final project is. Most hackathons also have company sponsored topics (eg: American Airlines, Google Cloud, etc) that are judged on a specific tech stack and/or theme. These prize categories can help you narrow down your idea to something doable and cool!
Something that a lot a teams struggle with is deciding on a common idea with their team. I like having everyone in my team pool together their ideas and write them down. This also leads to the side benefit of bouncing off the different ideas and compile a solid list. During this phase, it can be beneficial to think about specific problems that you and/or people you know face. The best ideas I've seen have typically been aimed at solving some sort of problem, whether that be wanting to play songs at the same time with friends or wanting to pair people up in airplane seats based on personality.
Naturally, some of your ideas won't be very feasible, others will have a lot of potential. Go through your list and remove the ideas that your team doesn't like. Something that's very important is that everyone on the team is passionate about your idea and likes it. You won't be able to work well together if people aren't happy with your final idea. Also think about how doable the idea is. If everyone on your team needs to learn some new technology for this project, then it probably isn't too practical to implement. It's much easier to create a minimal viable product (MVP) and then add on features and embellishments later than it is to try to narrow a project down and cut out important features later on.
After you have narrowed down your ideas to 2 or 3 ideas that everyone likes and thinks is doable, it can be helpful to make a pros and cons list. Part of this process can include looking into the technology you will need and start planning out how you would make each project. This can expose hidden complexities that may help narrow down the choices. Sometimes all your ideas are equally good and in that case, just pick an idea. Flip a coin if needed! Remember that you can always change your project, and that there is no pressure to do anything :)
Hopefully this helps your team get started with an idea. The rest of this guide is for you to get started on the technical aspects of your project. If you know you need a certain technology but don't know where to start on the technical side, check out some of the other sections for guides on learning. Happy hacking!