One of our favorite family things to do is go hiking in the woods. The kids love it, the dogs love it, and we bring marshmallows. At a little more than $1 a bag, it is a great day for your $$!
If you are lucky enough to live near a AAA ballpark, I highly recommend it. Ladies, before you shout, "But I don't like baseball" think about this: cheap tickets, fresh air, yummy (if overpriced) comfort food, and your husband and son thinking you're cool. Give it a shot.
Here's another freebie-<p>Depending on where you live, find a very small local airport (there are lots of them all over the country). Usually the parking is free, and you can find a place to sit an watch the airplanes land/takeoff which the kids love. If your H is into stuff like that, he may enjoy looking at the planes, etc. too. Talk to the people about taking flying lessons (you don't actually have to do it!). Lots of time they have inexpensive little restaurants in them, too, or a place to picnic.