If funds are low, activities as simple as a family bike ride, an exhilarating game of capture the flag or a no-holds-barred snowball fight with the neighbors could do the trick. Valentine's Day is just shouting to be reclaimed from card and candy makers, and what better place to start than right in your own back yard!
If the weather is too frosty in your area, consider a game of hide and go seek around the house or a scavenger hunt. Read books and comics together, or embark on marathon finger painting sessions. Perhaps the old toy chest needs a fresh coat of paint and a little flair, so let the kids have at it with your enthusiastic support.
When was the last time the kids put on a play or concert for mom and dad? Especially with games like Rock Band and Guitar Hero, there's sure to be a performance waiting to happen.