It really depends how much you like to hike. I could easily spend 3-4 days there just doing different day hikes. If you want other activities, look into white-water rafting, horseback riding, and boat rentals/rides on lakes in the park (assuming they're open this season). As for top hikes, Hidden Lake is a no-brainer as the best short, family-friendly hike, and once you're there at Logan Pass where it leaves from, definitely walk out at least 20-30 minutes on the Highline Trail. This is assuming you can find parking, as in normal years, parking at Logan Pass can be a nightmare unless you get there really early or late in the day.
The Many Glacier area is an absolute must just to see the view from the hotel, and I'd highly recommend the drive to Two Medicine to see the view there as well (the boat ride on the lake is really relaxing). And if you have passports, a leisurely drive up to the Canadian side of the park (Waterton) is really pretty as well.
Another popular short hike (4 miles RT) is Avalanche Lake, though if you have to choose, Hidden is better (shorter, prettier, and almost guaranteed close-up goat sightings). If you're up for one or two longer hikes (8-10 miles RT), the top 5 would be Grinnell Lake (can be knocked down to 7.5 miles RT if you take the boat ride to cut off the beginning of it), Iceberg Lake, Cracker Lake (not as pretty throughout the hike but a jawdropping destination at the end), the full Highline Trail, and Siyeh Pass (my personal favorite, but you can't go wrong with any). And if you really want to push yourselves, I once hiked the 18ish mile Pitamakan-Dawson loop on my birthday as my present to myself, and it was the best hike I've ever done.
As you can tell, I really love this place! You didn't ask about lodging, but that will be one of your most important decisions. Please don't make the mistake of staying outside the park in the big chain hotels on the west side (Whitefish, Columbia Falls, West Glacier). You'll be 60-150 minutes each way (!) from all the best scenery in Glacier and will spend far too much of your valuable time waiting in traffic. Best is if you can find in-park lodging on the east side, ideally Many Glacier, Swiftcurrent, or Rising Sun (and have reasonable expectations of in-park lodging, you're paying for the location). Any more questions, just ask, and hope you love it as much as I do!