You could go out and buy Battle Ship or Connect Four (game systems versions are now available) or you could just play them and host of other games with your grandkids using just pencils and paper. It is sort of like discovering that pancakes can be made without a mix! Who knew!
The advantage of these games is you get stuck waiting somewhere for some reson you can keep the kids amused and perhaps even patient.
Another rainy day sanity saver suggestion brought to you by A Second Cup.