Glacier N.P. is a must see if you havn't seen it before. Unfortunatly Park regulations and employees have changed a great deal over the years. Permits are required for almost everything imaginable and park officials aren't as friendly as before. Some can be downright cranky.
If you are able to do hiking, I would recomend the Highline trail that runs from Logan Pass to the loop. It is an all day hike, but worth it.
Winter travel through the Smith Valley can be a danger due to blizzards that tend to blow through. Another warning would be to environmentalists: If you are determined to come to Montana, Missoula would be more to your liking.
If you are coming in June, bring rain clothes.
Photo Equipment: Don't forget your camera!
Camping/Beach/Outdoor Gear: Make sure your tent is waterproof. Especially if coming in June.
Miscellaneous: If you are coming to visit the Park July and Aug. are the best months.