Bake cookies, let the kids dump the ingredients into the bowl, and turn the mixer on/stir with spoons
Build a fort of blankets and pillows in the living room, read stories under the fort
Make smores over the stove
Play hide and seek in the house with the lights off
Homemake ice cream, marshmallows, doughnuts, soft pretzels, or something that you like that you think simply comes from a store
Get a large roll of paper to spread over the floor and draw a huge mural
Find local craft, 4H, or ag/county fairs
Use cardboard to build a fort in the backyard
Have a "dance party"
Act out a favorite story with improvised costumes and props
Have a "move night" where everyone stays up late, eats popcorn and snacks and watches a favorite movie
Play in the mud/dirt/leaves/snow
In-house "Nerf" wars, or other non-injurous projectiles (have also used stuffed animals)
Find local minor league, non-professional sports teams to watch (typically a much more affordable way to enjoy sports)
Dig out a garden bed (let kids help with sandbox shovels, trowels, etc as age appropriate), let kids pick seeds they want to plant
Build a bird feeder, set it up by a window; find a bird field guide at the library, and see what sort of visitors keep coming back
Write up a scavenger hunt and take it to a public place and have the kids check off the things they find
Go "snipe hunting"
Bang spoons, pots, pans, etc, note that different materials make different sounds (wood, metal, pyrex, plastic, etc)
Host a formal "tea party" either with just your family or with friends
Is there such thing as Kids geocaching? Because if there isn't you should start one. Buried treasure is exciting!
Have a themed day, and do everything that day according to theme (pirates, knights, monsters, butterflies, cowboys, etc) For pirates I would serve "Marshmallow Mateys" for breakfast, fish sticks for lunch and a chicken vegetable soup for dinner loaded with round carrot "gold coins," we'd all wear hats and eye patches, and do silly pirate talk all day
***Ask the kids what they want to do and DO IT!*** It becomes an adventure in itself!