Banff, Jasper, Osoyoos, Vancouver, and Tofino have been my playgrounds for the past 40 years so here are my recommendations:
Banff and Jasper ought to be warm and sunny enough (daytime temps around 75-85F in mid-August) to hike in shorts and a T-shirt during the day. The mornings and evenings will be much cooler (think 45-55F night) so you'll need a hoodie or sweater and a pair of long pants. Mountain weather, though, is notoriously changeable so you should always pack a light-weight rain shell and fleece if you are hiking in the backcountry (down and cotton are pretty much useless in wet weather) because stormy weather with significant amounts of wind and rain can blow in very quickly. Both Banff and Jasper are pretty informal and you'll see gear-type clothing like fleece and hiking boots worn almost everywhere by everyone.
Osoyoos is a desert climate and will be HOT during the day (85-100F) so definitely shorts, sandals, sundresses, and bathing suits here. Evenings do cool off, but not like in the mountains. Temps can stay in the mid-eighties until the wee hours of the morning. Air conditioning is common and is usually set at freezing so bring a sweater for indoors. Rain would be unusual, but can happen, usually as a part of a storm cell moving quickly eastward, but most people huddle indoors until the system blows itself out.
Vancouver ought to be glorious in mid-August with warm, sunny days. People will be out in their summer shorts, skirts, and tops doing all kinds of athletic activities. The city vibe is casual, active wear with a twist of fashion. Lulelemon was born here and you'll see lots of it on the streets. My J Crew jacket works perfectly here because it provides enough rain resistance when used with an umbrella to handle summer showers quite nicely. Some places are air conditioned, but, because the temperatures are more moderate (65-75F), it usually isn't turned as low as in Osoyoos.
Tofino is right next to the Pacific Ocean and can be gloriously sunny or damp and foggy--you just take it as it is. Your mountain fleece and rain shell can come in handy here if the clouds and wind roll in, as will your long pants. If the weather is nice, you'll be in shorts and sandals with a sweater thrown over your shoulders. Tofino is definitely a laid-back, northern surfer, idiosyncratic town. Lululemon is not appreciated here unless it is old and ratty, but, on the other hand, the sky's the limit for your rain gear and surf board.
I have to run, but let me know if you want recommendations for places to stay and eat--or anything else--good wineries, shops, etc.
