Bath in the middle of the day with food colouring (just a drop). Hours of entertainment.

Big cardboard box. Crayons, blanket fort, hide and seek. Endless.

Indoor obstacle course with sofa cushions and tape on the floor.

Baking - even just measuring flour into bowls. They love it.

Toy rotation - bring out things they haven't seen in a month.

Library is free and has free story times. Underused.