Should the main menu also include a playable game, or is the focus purely on the menu itself? Also, are we allowed to experiment with a 3D UI, or should we stick to traditional 2D UI design?

Making a playable game is purely optional! You can even use an existing game project and just use this jam to work on the menu for it, for example. Also not only is there no requirement for the UI/menu to be 2D, but I encourage participants to experiment, so definitely feel free to make a 3D UI if you'd like!