Rocky Mountain National Park is the closest national park to Denver — about 90 minutes northwest of the city through the gateway town of Estes Park. Within its 415 square miles the land climbs from montane forest and meadow up past the treeline into genuine alpine tundra, with more than sixty peaks above 12,000 feet and a string of cold, clear lakes left behind by the glaciers that carved the valleys.
The signature drive is Trail Ridge Road, the highest continuous paved road in the United States, which crosses the park and tops out above 12,000 feet on a windswept ridge of tundra where the air is thin and the views run for a hundred miles. It is open only from roughly mid-May to mid-October; the rest of the year snow closes the high stretch. Lower down, the Bear Lake corridor is the busiest and most rewarding hiking hub, with a chain of glacial lakes — Bear, Nymph, Dream, Emerald — reachable on loops of anywhere from half a mile to a hard day.
Wildlife is reliable and close. Elk graze the meadows around Moraine Park and bugle through the autumn rut; moose haunt the willow flats on the wetter western side near Grand Lake; bighorn sheep work the rocky slopes, and marmots and pikas live among the tundra boulders up high. Come prepared for altitude and fast-changing weather — afternoon thunderstorms build quickly above the treeline, and you should be off the exposed ridges by early afternoon in summer.
The classic day trip from Denver is to drive up through Estes Park, walk one of the Bear Lake loops, have lunch in town, and return over Trail Ridge if the season allows. From May through October you must reserve a timed-entry permit in advance, so plan the date before you set out rather than turning up at the gate.