Where can I leave a vehicle for the entire summer? 

Calgary and Edmonton have safe, paid carparks, normally near the airport. It is always easier to hitchhike or find public transport towards larger cities to get back to your car at the end of the hike. It may be more expensive than leaving your car at a trailhead, but it is guaranteed to still be there when you get back, and you won’t have to pay for a shuttle to a distant trailhead.