Drumheller sits in the Red Deer River valley, where the Alberta prairie suddenly drops into striped badlands, dry coulees, and dinosaur-bearing rock. It is a small town, but it has a clear reason to exist as a destination: this is the easiest base for one of Canada's great fossil landscapes.
The essential stop is the Royal Tyrrell Museum, a serious palaeontology museum that is good even if dinosaurs are not usually your thing. Around it, Midland Provincial Park gives you short walks through the badlands, while the Hoodoos Trail shows the fragile stone columns that make this valley look so strange.
Drumheller is seasonal and spread out. Downtown is walkable, but the best sights sit along the river valley and the old mining roads, so a car makes the day much better. Come for the museum, leave time for the landscape, and do not treat the town as only a roadside stop from Calgary.