The best route to avoid mountain grades is to take I-90 to Livingston MT, then go south towards Gardiner and the north entrance to the Park. That highway follows the Yellowstone River and is quite scenic too. You could exit the park by going over to West Yellowstone and head north towards Bozeman and then be on I-90. But another way is to leave the park by the south entrance and visit the Tetons, from the Moran area take the highway to Dubois or continue south towards Rock Springs, this alternative would let you see a lot of Wyoming on your way home too.