Educational Travel on a Shoestring: Frugal Family Fun & Learning Away From Home by Judith Waite Allee and Melissa Morgan is such a great book to have on hand for reference. I highly recommend it.
They share a myriad of information ranging from free field trips/day trips to raising funds for service oriented trips. There is info on researching your destination, camping, rving, frugal food, lodging, home exchange and even a great section on how to make lifestyle changes to help make your travel goals a reality.
And the authors are home schooling moms so they share in a way that home schoolers can definitely appreciate. They also wrote a book called Homeschooling on a Shoestring: A Jam-Packed Guide for Parents -- check them out!