Mt Assiniboine Provincial Park is an absolute gem of the Canadian Rockies. The area as a whole is incredibly photogenic and I cannot recommend it enough.
Since there are no roads into the park, the only way in is either on foot or by helicopter. Flying in is certainly easier but you miss out on a lot of what this park has to offer. There are two ways to hike into the park and these are either via the Mount Shark trailhead or Sunshine Village. I hiked through Sunshine Village so that's what I'll be talking about here.
The Sunshine Village Lodge is a 10 mile drive from Banff. From the parking lot you can either take the gondola up to the village or hike 3 miles of brutal incline. The gondola is only running from June 28th to September 8th in the summer so plan accordingly. I backpacked this before peak season but I was lucky enough to catch a ride from a village employee but I'm sure that this is rare.
From Sunshine, the hike to Mt Assiniboine is broken up by three campgrounds: Howard Douglas Lake, Porcupine, and Og Lake. Howard Douglas Lake is only 4 miles from Sunshine Village so it's not really worth stopping for. Porcupine campground is about 9 miles from Sunshine Village and this is where I made my first stop. You could probably continue on all the way to Og Lake but I had to hike through snow for most of those 9 miles and there was no way I was going to hike any further. Porcupine is a pretty boring campground looks wise. There's not a lot there except for a handful of tent pads and some bear boxes. There is also a nice stream where you can filter water for the rest of your hike.
After Porcupine, it is another 6 or so miles to Og Lake. Og Lake was my favorite campground of the trip and on a clear day the view from the lake shore is amazing. Keep in mind that you do have to reserve a spot online at this campground and at Lake Magog and it best to do this sooner rather than later because spots are very limited.

View from Og Lake
If you've got time it is worth while to hike up to the top of the rocks on the left side of the lake to get a different look at the valley you've just hiked through.

From this point on the views in every direction are incredible. From the Og Lake campground it is an easy 4 mile hike to the Lake Magog campground and the Mt Assiniboine Lodge. I'd recommend you stay here for a couple nights to fully take advantage of everything this area has to offer. I'd recommend hiking to Nub Peak and Cerulean and Sunburst Lakes. Both provide great unique views of the mountain and neither are very far from Lake Magog.
Trip Details:
Distance: 35 Miles Round Trip
Elevation Gain: 6,300 ft (the entire hike is above 6,500 ft)