I don’t own a SXS but have been on several rides with SXS owners using my 4 wheeler. As bladepuller mentioned, a proper toolkit.Figure out your lugnuts, electrical connections, things you might have to remove and throw a real wrench/ socket set that meets those needs into a kit. I try to keep mine minimal- maybe I’ll need to replace a battery or empty my crank case and put new oil in (after submerging), or remove a wheel, etc.
That kit will depend on how exactly you plan to use or abuse your ride; more abuse (I’m in that camp) = more tools might be necessary. The small kits that come with machines from the factory, if they even come with anymore, are usually hardly usable in a real life scenario. They look neat but they aren’t great.
I would also recommend a small bottle of slime (not fix a flat) or a tire plug kit, a small jump pack/ compressor unit, and absolutely a tow rope no matter how you use your machine. Flats and tows will happen regardless of how nice you drive; for example, every time we ride the soo line trails someone gets a flat from an ancient railroad spike.
I have a kit setup that fits in the trunk I have on the back of my wheeler and I adjust it as needed. I make adjustments depending on what we plan to do (mudding vs. trail riding, how long we’ll be out, etc).
One thing that is often overlooked is water. bring water in your cooler or a bottle, but also a lifestraw or swayer filter can be an absolute lifesaver, allowing you to drink from a lake in a major pinch. been there, used them, the worst thing ever is running out of drinkable water while miles away from your next stop on a hot and dusty day.