Indoor scavenger hunts are super fun and easy to do because all you need to do is print out a list like this one and have your kids run around the house until they find all the items.
Spice it up by having each kid pick a book with a specific theme (like sharing or money or something about sleep) and take turns reading out loud.
Any toy works for this, but Clicks, Lego, or Magnatiles are great for this. Have them sort it all by color (either in piles or in towers).
These calm down jars were super fun to make and fun to have. It calls for pretty basic materials, and you can use your imagination to adapt them to whatever you have. This is the actual one we made.