If one of them is decrepit (my dictionary told me that), you may lengthen its life as a superb art stamp
Cut a piece of the desired size and glue it to a wooden handle. Easy as pie. You may use it like a normal rubber stamp with the stamp pad of your choice.
I made a border design with that stamp, but the possibilities for creating backgrounds are endless.
PS: Collect old hot water bags and share with stamping friends. You may swap different patterns with each other. And no one really needs the whole rubber of the bag. One bag makes a lot of stamps. Swapping extends your selection. Have fun!