We are planning on coming to Big Sky last week in October. What are some must sees,?

Many Glaciers

Flathead lake is a must!! BEAUTIFUL!!!

If you are coming from the south or east on I-90 there is Mt. Rushmore near Rapid City, SD that is a must see too.

East and west side of the park. Both very different in own ways and activities. Most wildlife up by Many Glacier entrance.

Glacier, Yellowstone, the Bison Range, Flathead lake, the question is a bit ambitious as the state is huge, but that should get you started.