My favourite outdoor ceremony space in Banff is Tunnel Mountain Reservoir that you can reserve through the Town of Banff. Tunnel Mountain Meadow offers stunning mountain views for your guests to enjoy and a location that is off the tour bus route so you can expect more privacy than typical outdoor ceremony venues. I love hearing guests excited reactions when they step into the meadow and are welcomed by the mountain views.
If you are staying at Buffalo Mountain Lodge you can simply walk across the road to the meadow which is lovely.
It should be noted that Tunnel Mountain Reservoir has no shelter or bathrooms on site so you'll want to take that into consideration and make sure you have a back-up indoor ceremony site that you love in case mother nature brings rain on your wedding day. Being a mountain meadow there is also the potential for wind so if you do decide to have any kind of decor like flower vases on pedestals make sure that they are weighted down more than you would think you need because I've seen some big decor go down quickly in a surprise wind gust. For brides who are wearing a long veil the wind can create some beautiful veil movement for photos but also may drive you nuts so make sure you choose a hair style and veil that will work well in mountain wind so you aren't fighting with it during your ceremony.