The Cave & Basin National Historic Site (parks.canada.ca; adult/child $9/free), 1.6km west of the town center, is where Banff's tourism all started. The Canadian National Park system was effectively born at these hot springs, discovered accidentally by three Canadian Pacific Railway employees on their day off in 1883 (though known to Indigenous Peoples for 10,000 years). The springs quickly spurred a flurry of private businesses offering facilities for bathers to enjoy the then-trendy thermal treatments. To avert an environmental catastrophe, the government stepped in, declaring Banff Canada's first national park in order to preserve the springs.
While you can't bathe here anymore, the place has been preserved as a museum and National Historic Site allowing access to the shadowy cave, the original outdoor bathing pool, and a terrace that covers the once-popular lower mineral-springs pool that welcomed spa-seeking visitors for 78 years. It continues to be a sacred site for the region's Indigenous People. An adjacent boardwalk with interpretive signage leads uphill to additional springs and the cave's upper vent. Budget 30 to 45 minutes for your visit.