Although there is much to do and see on both sides of the park, we prefer the east side over the west side; there is more to do, including Many Glacier and Two Medicine.
On the west side, we would stay at the Apgar campground (no hook-ups), in the park. For full hook-ups, we would choose the KOA outside the park. On the East side, we stay at St. Mary KOA. None of the park campgrounds on the east side have campsites big enough for our rig.
Clicking on a picture will open a larger version in a new tab or window.Here is a view across McDonald Lake, on the west side of the park.
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Some popular hikes:
Avalanche Lake
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Hidden Lake (probably the most popular hike)
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Highline Trail - spectacular scenery but not for those afraid of heights.
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Enjoy your trip,
Tom
On edit: Cleaned up the links so that clicking on the pictures works.