is, without a doubt, chewing.
These little darlings can chew for hours. Whether it's a steak bone or a rawhide chew, they will be amused for a good long time. I always make sure I have toys they can chew as well as "pig ears" and other items that are made for dogs. It's good for their teeth and, most importantly, it saves your furniture! Do not think these cute little dolls are beyond eying up and going after your cabinet edges, tables, shoes, purses, area rugs (a favorite "toy"- they love working the wool pieces out of the matting) and couches. If they don't have plenty of toys and dog chews, look out...your things are on the hit list.
So for the sake of your sanity and the health of your pets (since I seriously doubt that wood stain has any health benefits), please make sure you are well stocked with things the Shitzu loves to chew.