Summer has arrived in Colorado's high country, and a quartet of high-altitude resorts in Summit County are cranking up for a bustling summer at four of the Rocky Mountain's busiest mountains.

Typically opening mid-June, standard warm-weather attractions include scenic chairlift or gondola rides, coasters and slides, hiking and mountain biking, disc golf, climbing walls, and music, food and drink. Expect that some labor-intensive activities like ziplining may have hours curtailed or, in the case of Breckenridge and Vail, be shut down.

At Arapahoe Basin, Summit County's only via ferreta has half- and full-day guided tours on the "iron way" -- a series of iron rungs fixed in the rock face -- that begin at 12,000 feet of elevation, and top out at the 13,000-foot summit on West Ridge. Near the base, the Aerial Adventure Park runs ziplines, swings, and balancing acts through old-growth forest.

Down the road at Keystone, tubing lives on into the summer, as crews pile up leftover snow atop Dercum Mountain (11,640). Lanes typically stay open into July, 10 a.m to 2 p.m. You must buy a ride up the gondola to the summit, and pre-pay by the run for tubing. Plenty of other activities at mountain-top Adventure Point.

On the other side of Dillon Reservoir, the force of gravity prevails at Breckenridge in the summertime. The Gold Runner Alpine Coaster pitches down 2,500 feet on elevated tracks, with two 360-degree loops and a top speed of 27 mph. The resort also has three alpine slides that run 2,600 feet down concave tracks -- with manual braking.

And to the west, Copper Mountain's summer starts in the base area. Check out bumper boats on West Lake, and a challenging go-kart track. The Woodward Wrecktangle presents a dozen obstacles to overcome in the Woodward Copper extreme sport complex. All-day summer pass includes Wrecktangle, unlimited rides up the chairlift, and limited shots at other activities.

The cheapest activities at any of the resorts are hiking and mountain biking. Check Forest Service regulations for e-bikes. Most have designated trails.

Many have stared amazed as rock climbers wind their way up seemingly vertical cliffs in the Western mountains, but few take the plunge. But now, the advent of via ferratas -- "iron roads" -- has brought the holds, cracks, caribeeners and safety of rock climbing to a wider audience.

Rock climbing is a specialized sport, with its own equipment and techniques. But European-based via ferratas have begun to spring up in the U.S. mountain country to make the sport more accessible to more people.