Yosemite might just be the gem in the crown of the whole national park system. Over 800 miles of trails wind past 3,000-foot granite cliffs, rocketing waterfalls, and groves of giant Sequoias. The immensity of the Valley itself is hard to describe. Suffice it to say you'll never feel smaller than while camping beneath the towering pines and time-hewn rock of this classic national park. Another benefit to the valley's depth: At dawn, the deep shadows inside the valley make the rock look purple, while the sun's first rays leave the rim glowing red-gold. Bring your camera.
Yosemite National Park gets packed during the summer months. You can avoid them by staying away from the Valley proper and adventuring amid the granite domes elsewhere (like in Tuolumne Meadows) instead.
Whatever your plans, try to organize your trip around the reliable (and free) shuttle service, operated by the park year-round. Check out this page for details on public transportation to the park, and within the park itself. Buses run into the park from various nearby cities, as well, which can be a convenient and economical workaround for the frequent parking shortages.
While there, don't miss the vistas from Tunnel View on your way into the Yosemite Valley, as well as the lookouts at Washburn Point and Glacier Point, which offer views of Yosemite Valley, Half Dome and Yosemite's high country.
The Half Dome Loop via the John Muir Trail is a classic, and exceptionally popular. For that reason, day-hikers will need to enter the lottery system to make the ascent. (Note: the final push to the top of Half Dome via the Cables Route is a bit exposed. Bring shoes and gloves with a good grip.)
The park is beautiful all year, but beware of winter conditions, chain requirements, and closed roads in the winter months. While the Yosemite Valley and Wawona areas can be accessed by car year-round, Tioga, Glacier Point and Mariposa Grove Road close for the winter, generally between November to May depending on snow conditions.
Waterfalls flow strongest in the spring, and wildflowers bloom in June. Or, visit in fall to take advantage of cooler temps and smaller crowds. In the winter, you can break out your snowshoes or cross-country skis on many of the trails. Badger Pass is a particularly popular winter destination.
Like most national parks, fees are required to visit Yosemite, and can be purchased either upon arrival, or in advance, here.
Wilderness permits are required for any overnight stay in the Yosemite Wilderness. Details can be found here. Day-use permits are not required on trails, except for Half Dome.
For backpacking, it's difficult to get the required permits for the John Muir Trail, which traditionally starts in the valley. You can't go wrong with any of the trails in the national park, though, and most others are in much less demand.
Dogs are allowed on a leash no more than six feet long, and only in developed areas, fully paved roads, sidewalks and campgrounds (excluding walk-in campgrounds, like Camp 4). Get more information on pets in Yosemite, here.