I would say either Grinnell Glacier or the Highline Trail. Grinnell Glacier takes you past 3 lakes and up to the meltwater below the glacier and is stunning every step of the way. The Highline Trail leads you from Logan's Pass visitors center all the way along the Garden Wall with non-stop ever changing views all day long. You can also make it a through hike going through Granite Park Chalet and ending at the loop. Then take the shuttle back to the pass. I also recommend adding on the Grinnell Glacier Overlook if you can.