Aftermarket running boards.
Stock one have a huge outer rails which puts strain on your ankles and makes it harder to move around on. Stock boards hold snow.
Tube style give you a flatter surface to dance around on. Clears snow way better which can be an extra 1-10lbs depending on snow.
Downside- $500+ and 10 hrs to install.
If we're talking spare part then I would say air intake screens. One tree and your done. Easy swap if you have a spare but if you don't it's a trip killer.
A backup sled, unit or machine, if your on a Doo or a Poo.....yes, its a part, kinda, its a collection of parts, all held together....somewhat, to make one part. LOL
Kinda like a collage or kaleidoscope mixed into one.
While it's not a must have, with 2 wheel rear axles, if a bearing in one of the idlers packs it in, you're done. I've been carrying a spare bearing in the trailer on a regular basis and over the past 12-15 years we've used it atleast 5 times.
Proper tools and spare belts, plugs, tow straps, ect. I think I pull out the tools at least once a trip whether it's for my group or lending a hand to another group. Need to be prepared.
Aftermarket parts, tether is a must have