3D Printing

If it doesn't come with the 3d printer, a pair of flush cutters is insanely useful. Just be careful with them, especially if you have a cheap pair. Probably wear eye protection.

If you think you would find them useful, there are also filaments at different levels of softness, bounciness, and foaming variants of those. Particularly useful for the soft ones as you can get different levels of softness by changing printing temperature. For any it helps to decrease weight.

For 3D modeling software, Fusion is good but annoying to obtain, Onshape is good but has a non-commercial license for the free version (and makes all of your files public), Freecad is FOSS, decent but not quite as good, Blender is good for detailed or sculpted things that are more art-y (although it's often very difficult to achieve certain shapes that are easy in actual CAD software)

My modelling advice is to keep in mind where supports will go, what places can be bridged, what details the printer can achieve, what axis the vertical should go on (for strength)