When Caitlin was about seven or eight months old, I bought a radio flyer red wagon at a garage sale. We did a lot of things with that wagon as she grew. Besides going for walks, we put water in the wagon to play in. We put all kinds of sponges, rocks, leaves and anything else we could think of just to feel the way they felt or watch the way they reacted in the water. She did too.
Sensory stuff is the thing that fun is made of to a baby or small toddler.
We can enjoy it also. Taking the time to explore, feel and experience their world, I am convinced, makes a huge difference in the relationship that you are building. Play with your grandchildren. Not as a chore but with the idea of what could develop inside your life too.