My version of a rainy day family activity? Marathon monopoly and baking stuff that's really fattening.
Rainy Day Mess
1 large bowl
several boxes of cheap cornstarch
tablecloth you don't want any more
small, washable objects to submerge (keys, marbles, coins, bottle caps, mismatched doll parts)
industrial strength soap
Dump boxes of cornstarch in the bowl. Add between 1 and 1 1/2 times as much water as cornstarch. Mix with your hands until it's nice and goopy. If you use less water, the glop will be slightly firm, like shiny chalky jello. If you add more water, it's just gooey and slimy. Drop in as much food color as you want. Mix again, making sure to slop a good portion onto the table and everybody's clothes while you're at it. This step can last as long as an hour.
When your audience begins to get bored, start dropping objects into the bowl. They will sink into oblivion. Kids will then dive in to try and find the objects, chanting loudly with each new discovery. They will then delightedly drop objects back in and start over. Seriously, it totally works.
After a few more hours have passed, try to end activity when all objects are out of the goop. Otherwise you will either have to fish them out yourself or toss them. Put kids into bathtub with industrial strength soap. Bundle up clothes and bring immediately to laundry room. Tip contents of bowl into waterproof trash bag. Roll up tablecloth and add to trash. Wait a day, then vacuum up caked, dry residue of multiple enthusiastic drips from rug. Hide any remaining cornstarch and food coloring from husband.