Sand is good. I used some of it commuting in the morning when I lived farther north. A bag is a good way of carrying it because the bag itself is useful. Hint: Don't use beach sand because the salt will eat your vehicle if you keep it inside. Course builders sand, with sharp grains about 1/16", is ideal.

Another good thing to keep handy is a piece of plywood to keep your jack from sinking in to snow or mud. One foot square is a handy size. A second larger piece of plywood, or a piece of heavy canvas or outdoor carpet, gives you something to kneel and lay down on and helps keep the tools and parts from sinking in the mud and crud.