Idaho Springs is accessible year-round, and each season brings a different crowd and character. Summer (June through August) is the busiest period: the mountain air stays mild and cool, Clear Creek runs at good whitewater levels through July, and the Mount Evans Scenic Byway is open to the summit. Expect plenty of traffic on I-70 on summer weekends as Denver residents head up to escape the heat.
Autumn is quieter and arguably the most pleasant time to visit. Aspen groves in the surrounding mountains turn gold in late September and early October, and the crowds thin after Labor Day. Winter brings ski traffic from the resorts further west — Idaho Springs sits on the main artery to Breckenridge, Keystone, and Arapahoe Basin — so the restaurants and shops stay busy on weekends. Spring is mud season at higher elevations, but the town itself is fine, and prices are low.
The hot springs at Indian Springs Resort are appealing in any season, though soaking in a geothermal pool has obvious appeal in winter. The mine tours at Argo run year-round except for a brief closure period in late fall.