I keep a hand towel in my car. Reason #1 ---when my mom was in the car one day she but into an apple and the juice squirted all over my brand new vachetta on my neo....ended well but that's how the towel got here. I also use it to protect my bags on sunny days. Just grow it in top and I'm covered.
I also keep a spare dust bag in my arm rest. Whenever I have to leave my bag in the car, I tuck it in there and hide it under the seat. I don't want it to be seen but I also don't want it on the bare floor.
I also have chicobags to protect them from the rain.