It does not rain much here and definitely no snow, so we have these light duty floor mats: ToughPRO.
We just installed the rear door sill protectors from RPM Tesla. Next time though, I would just buy a piece of XPEL or any other PPF film and cut to size. The rear door sills are larger than most doors and are the perfect place for little feet to step on.
We also have the luggage protector from RPM.
In the trunk, I plan on keeping a supply of plastic bags, towels, and wet wipes.
I also keep a large piece of flannel fabric in the trunk. I use it as a blanket for those times it gets cold at night and we forgot to bring jackets for the kids. I also use it to cover their car seats when we park under the sun. I might need to get a piece for my own seat too. The seats in the M3 get so hot!