The Highline stretch from Logan Pass to Granite Park is a hiker freeway. It will be packed with people in mid-August. It is still quite beautiful, but I only hike that section if I have family or friends in town who want to see the park. Personally, I don't mind the crowds, as I've learned to accept their presence as a part of the democritizing power of wilderness, but it is not the deep wild experience that other parts of the park can be.
As to Dave's suggestion of hiking up Swiftcurrent Pass, down to Mineral Creek, and back up to Flattop Mountain, it will be a mixed experience. Likely, once you get a couple of miles from the Many Glacier Hotel, day-hiker presence will drop off dramatically, especially once you begin ascending. From Granite Park down to near the Loop (a bit before Mineral Creek), expect a fair amount of trail traffic. While I haven't actually made it up to Flattop Mountain, I would expect that once you leave the main trail from the Loop to Granite Park, you would have quite a bit of solitude.
My only issue is that if you go this route, you will miss out on the fantastic experience that is the Northern Highline. Dayhikers rarely venture past Granite Park Chalet, so the trail beyond is often quite empty. The miles are significantly longer, but the elevation change is much more mild. Frankly, either Flattop Mountain or the Northern Highline will be fantastic routes, and any hemming and hawing back and forth about either one just underscores how good your choices actually are.
For first time visitors--or those who only get out here once every few years--the Northern Highline is hard to beat. As a plus, you would also enjoy much of the Flattop Trail by heading to the Fifty Mountain trail junction and then going over Flattop campground. As a bonus, if you go that way, take the spur trail to Ahern Pass. It's relatively short (less than a mile one way), and you'll look down some impressive cliffs onto Helen Lake. It's one of the more tucked away gems along the trail.
Honestly, no matter which route you take, you can't make a wrong decision. It's a fantastic itinerary.