Hey, if you don't finish the game it's completely ok this is all about fun.
General advice on jams is that you don't need to have it being polished but just a type of prototype of the idea. The art could be very simple and done quickly or you can use an asset. I would recommend that you think of some ideas before you start the jam and when creating levels write it on paper before doing it on the game engine to save time.
Other than that there isn't much else and it depends what you are the best on because you could be more of an artist or a level designer and so you could focus on those more.