"banff area, not too built up, near a lake"
good luck, this does not exist. It being in the national park means most of the campgrounds are built up, require a park pass, and put you right next to your neighbours RV and his 5 kids.
Some suggestions are:
Kananaskis area. Search for "Lower Kananaskis Lake" there are plenty of campgrounds, and the place isnt too over run this early in the season. there are seasonal restrictions on the road, but past May 15 you will be good to go.
Cowboy Country. Plenty of good camping by the multiple rivers and such around Black Diamond and south of there. Chain Lakes also provides lake fishing, but its right next to a highway, I suggest you bring your fly rod (which I never owned when I lived in Sask, hahaha) as the season opens real soon/is open.
Since you are a prairie boy....PM me, ask me real nice, and Ill give you the place to go....im not putting it up here so some google search eventually finds it. It's not near Calgary, but is equal/less driving distance from you. Tonnes of lakes and rivers, in the mountains, no nearby towns, yet good roads to it. Best part is that it is NOT in a park, and you can camp wherever you please, or get in one of the many campsites there are which are secluded and "not built up". As a friend put it "It's Banff 80 years ago."