They were often recycled into a great karate training equipment known as ishi sashi.
In order to use them, one can practice basics or a kata slowly while holding these. Focus on the slow movement and breathing. This is why Sanchin is a very popular kata to do with these. Most of my students don't know Sanchin so I would suggest Seisan, Naihanchi Nidan, or basics.
Like many other pieces of hojo undo equipment, these are relatively low weight. I'd suggest 5 pounds for adults, and 3 for younger students.