For a park like Glacier, you should take a small backpack with you.

Wear a lightweight long-sleeved hiking shirt (covers you from the sun, keeps you more or less cool) and I like zip off pants that can change from shorts to long pants easily.

IN the pack, take a rain shell or poncho, and some kind of fleece that can keep you warmer.

And take a look at a good list of ten things to take on a day hike...make sure you have a really good reason to leave any of those things behind!
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