Here's a tip: pick a museum that usually charges admission! Some of the museums listed are free all the time. Which is great to know for other day excursions, but you may want to save "Museum Day" for a museum you couldn't or wouldn't go to otherwise. Make sure you print the pass so you can get the free admission.
Do you know of other venues you'd like to go to but can't afford? See if they have special discounts for certain days or times of year. Example: Fantastic Caverns in Springfield, MO just sent me an email notice that they are having "Homeschool Days" at the beginning of October. It will have reduced admissions for homeschoolers.