cushion/blanket forts! a whole imaginary kingdom with wars and romance and goblins and magic
dress up. (bonus: good time to clean out old/too small clothes) take pictures (kids love to see pictures of themselves being silly).
kitchen band. (if you can stand the noise) pots pans, spoons and spatulas. many glasses filled with different amounts of liquid to make different notes.
treasure hunt inside the house. using "hot" and "cold" for hints.
bathtub bubbles. dish soap and a couple inches of water in the tub (kids in the tub too!). use wire clothes hangers for different sized bubbles, some as big around as their little bodies
play "grocery store". take everthing out of the cabinets, give them a list (stuff to make pb&j's, cupcakes etc..) and let them "shop". bonus: good time to clean out and organize the cabinets.
reenact their favorite book with them, start to finish.
go through pictures with them. put them in order, telling the story of their birth and lives.