What's an affordable and fun family activity?? My answer would be anything outdoors, weather permitting. There are so many wonderful things to do outdoors with the family. Go to the garden center in your location and buy some plants and seeds. Showing the kids how to plant a garden, no matter how small, is a great lesson for them to learn plus it's Free with the exception of the cost of seeds or plants. They'll enjoy watching as they bud and grow.
Another fun activity is heading out to the local and state parks. There are tons of play equipment and places to picnic. Some state parks offer horseback riding, swimming pools, hiking, and even some have small recreational shops. Although not free, it's still an affordable option.
If you're looking for an absolutely FREE option, head to the backyard. Nothing is more fun than wresting in the yard, playing ball, and having a nice bonfire come evening. That's OUR favorite thing to do. Add some hotdogs and marshmallows and you've even got supper covered for cheap!