For rental, you should check out Banff Cycle. They have DeRosa Planet and Sk Pininfarina as well as Cervelo R3 for rental.http://banffcycle.com/rentals-2/
Ultimate Banff and Snowtips-Bactrax are two other options.http://www.ultimatebanff.comhttp://www.snowtips-bactrax.com/Full_Su ... Bikes.html
Those 3 are actually in Banff, if that is where you are staying. There are also some rental shops in Canmore.
The Banff Legacy Trail is nice. I think round trip from Banff to its end, it is around a 30 mile round trip ride. Depending on the time of day, it can get quite crowded in the summer with families and little kids riding the trail. But for a nice relaxing ride to enjoy the scenery, it is a nice ride. https://www.pc.gc.ca/en/pn-np/ab/banff/ ... age-Legacy
If you are looking for something else, where you can really open things up and not deal with other cyclists, I would take Vermillion Lakes Rd west our of town until you hit Legacy Trail. Then take that to Bow Valley Pkwy (1A) and go as far as you want. I believe Lake Louise is about 40 miles from Banff if you want to go there and back.
The Lake Minnewanka Scenic Dr is a nice loop and isn't far from downtown Banff either.
Have fun and enjoy. Banff is a beautiful place.