Grandchildren - What to do on those long wet days?

This is going to be different for different age groups, but we're always up for new ideas. One of our favourites (with grandchildren 5 and under) is a mountaineering climb (horizontally but...let's pretend) from the sitting room to the kitchen, throwing each other ropes (!?) stopping for drinks, helping each other over the rough bits, pulling each other when necessary! All other ideas welcome! A pity these days can occur even in Summer when they want to be outside!! They also like climbing over cushion 'mountains'.