DH and I spent one long day in Glacier two years ago. It was the middle of a heat wave, and 100 degrees in the afternoon.
We arrived in the west side of the park around 8, drove across the Going to the Sun Road (which is stunning), paddled for about an hour on St. Mary's Lake on the East Side, had lunch at the East Glacier Lodge, drove back on Highway 2, reentered the West Side of the Park, spent about two or three hours floating in our kayaks on Lake MacDonald, and were back in Kalispell having dinner by 8 or so that evening.
We were in Yellowstone earlier this week. We left Bozeman late (around 10), drove into the park through West Yellowstone on Highway 191 (just beautiful), spent about an hour and a half in the middle of
Disneyland like crowds at Old Faithful, and then looped around to the east end of the park and out on Highway 89. We got back to Bozeman about 7 that evening.
We weren't able to do any hiking because I was coming down with flu, but we did get out several times to look around. We encountered a bison on the road, and saw elk and deer.