A good start would be figuring out what your top 3-5 things would be and then trying to tie them all together.
Yellowstone is nice, takes a day or two to do properly. It is nearly a 400 mile drive from Yellowstone to Columbia Falls. A nice day trip from Columbia Falls is leave Columbia Falls and truck over to West Glacier and hit the Going to the Sun Road (usually open in July) to St Mary at the other end. Then take 89 through Browning (remember, don't stop
) and pick up Highway 2 going back west and do a quick stop at East Glacier to gawk at the lodge and have a huckleberry margarita. As you are headed back take a few minutes to stop at Goat Lick and possibly see the mountain goats on the cliff face licking up the minerals. Then just head back to Columbia Falls, possibly stopping in Hungry Horse to eat. That route will also put you on the top of Marias Pass, the continental divide.
bob