After a a full trip to Banff/Jasper, this (and Yoho NP) stand out as my favorite visit.
I saw this as more "hands on" for my young teens, meaning there was more to do than hikes and views -- which you know is important if you have young teens. The big grassy bluff of the Prince of Wales Lodge is open for all to run around and experience the winds coming up off the mighty view of Upper Waterton Lake looking deep into the United States. You can actually do the "lean-into-the-wind" stance when the winds are strong enough, which they were on this day. But walk away from the bluff and the winds diminish for a wonderful walk about the uncrowded and unpretentious lodge.
The townsite is right on the lake. It has a long rocky beach that you can get into the water or even just skip rocks. Not as typical of the other popular National Park lakes that we had visited. The tourist shops just stretch around two blocks, so you can indulge the family without it getting carried away. More of a local feel as well.
Be sure to drive the short Buffalo Paddock loop which is north of the Park entrance. A single roadside sign right at the turnoff from the highway coming in from Pincer Creek. You'll be sure to sight some bison feeding somewhere within the meandering gravel loop.
We arrived at Pincer Creek the night before and found very adequate lodging at Parkway Motel. Glad we did not try to find drop in camping at Waterton because it appeared that everything was full this weekday.
Lastly, this was the first time I ever had zero - I mean zero - wait crossing into the US border. Heading across Chief Mountain Border Crossing, we were at the east side of Glacier National Park in less than an hour. That's the only thing I can't guarantee, but otherwise I can guarantee that you'll love Waterton!