Books, books, and more books!
Listen to music and dance along.
Play “horse and wagon”- My oldest (10) initiated and runs this activity: cardboard box is the wagon, pile in some blankets and a pillow, tie jumprope around box and around “horses'” waists, and “drive” a light-weight sibling around the house.
(Sometimes the “passenger” falls asleep on the journey.)
Big kids spread old, large blanket on floor and shlep little kids around on it.
Make puppet shows! I once bought a puppet theater with puppets; very good $30 investment.
Turn the shower stall into anything- library, airplane, or just a little, fun place
(on condition that all props be returned to places of origin).
Hide and seek. Tag.
Anything, but housework!