A small Persian type area rug seems to be a popular choice. Partially because they are not too bulky i.e. not much depth/height, and are colorful.
When underway, it can be rolled up and stowed out of the way. I put some non slip rubberized backing on mine to keep it from sliding underfoot. On a varnished cabin sole, a small area rug can be dangerous without some type of non slip material under it.