For the full experience of Glacier, bringing along family passports will allow you to travel through the Canadian border. It's not a must but could come in handy if you have extra time. Food, water, small first aid kit and maps (you can't rely on your phone or GPS in some of the areas) are wise to bring. Bear spray is a must. Many hotels in the area offer it for free. Ask at the front desk.
Ask the ranger at entry points if any areas have had reports of grizzly bears. Avoid those spots. Don't wander onto roads without specific signs. We found an intriguing side-road off a trail near Lake McDonald and decided to be adventurous and drive down it. Thirty minutes later, after driving through rocks on a one-lane road, we found ourselves in the middle of nowhere and had to back-track.
Mornings or late afternoons are best for avoiding crowds. Weekdays are better than weekends. Road construction may slow you down. Expect to stop. Unroll the window and enjoy the sounds and solitude of the scenery.
The most popular and brilliant pass, Going-to-the-Sun Road, may be closed as late as June and as early as September due to weather conditions and road construction.
Entry to the park is $20 per vehicle. We purchased an $80 National Park Pass that is valid for all U.S. parks for a year. You can get combined passes for Glacier, the Grand Tetons and Yellowstone for a reduced rate. Many hotels have free bear spray at the front desk since it cannot be transported on a plane. If you aren't flying, ordering ahead of time can save as well.