It's called the Crown of the Continent for a reason. At approximately a 5.5 hour drive from Sage Lodge, this park is a must-see if you are planning on spending extended time in Montana. The East side of the park offers the best views of the mountains and lakes although it is a bit more rustic than the West side, which is built up with more tourist attractions/restaurants/etc. We recommend driving the Going-to-the-Sun road, visiting Lake McDonald, Two Medicine, and Saint Mary Lake.
Our recommended must-sees: Mammoth Hot Springs, the Grand Prismatic Spring, and Old Faithful. From the north entrance, you can reasonably visit all within half a day. If you're feeling adventurous, Grand Teton National Park is a short drive away!