Today marks the 100th anniversary of the National Park Service, and to celebrate the occasion – and inspire the next generation of visitors to visit America's prized parks and historic sites – the parks will be offering free admission through Sunday. So, now is the perfect time to pile the family into the car, and head to Utah to embark on a Grand Circle Tour of top national parks. Popularized in the 1920s, when the Union Pacific Railroad created the route with the then newly developed National Park Service to encourage visitors to get outside and admire the plateaus and peaks of southern Utah and northern Arizona, this iconic route is not to be missed. If you're ready to embark on an epic road trip, packed with striking scenery, storied sites and plenty of time in great outdoors, here's how to plan a memorable tour of four inspiring national parks. [See: 15 Must-Visit National Park Attractions].Zion National ParkUtah's first national park – established in 1919 – Zion sprawls across the southwestern Utah desert. There are many regions within Zion National Park, but the park is best known for its dramatic red rock cliffs. Visitors can stroll along the Riverside Walk hiking trail and wade into the water to cool off or, for a thrill, climb to top of Angels Landing for striking 360-degree views of the valley below. Not up for hiking? Take a scenic drive through the Zion-Mount Carmel Tunnel or the less-visited Kolob Canyons section of the park. Keep in mind cars are not allowed in the main Zion Canyon during the busy spring and summer months, when visitors must ride the free shuttle system to get to the hiking trailheads. However, visitors can drive their own cars through the Zion Canyon as the crowds diminish, from November to March. [See: 7 Affordable Ways to Experience America's National Parks].Cedar Breaks National MonumentA hidden gem in the mountains above Cedar City, Utah, Cedar Breaks National Monument is a fantastic way to get away from the crowds of Zion and Bryce Canyon. Cedar Breaks' prominent feature is a majestic 2,000-foot-deep amphitheater of reddish-orange rock carved with intricate fissures, and hoodoos (skinny spires). Situated at 10,000 feet elevation, Cedar Breaks offers much cooler visiting conditions than Zion National Park's desert heat. Plus, nights are punctuated by dazzling stars speckling clear skies. Be sure to stick around after sunset when park rangers offer lessons about our solar system via telescope. And don't forget to take notice of the trees along Cedar Breaks' rim – the bristlecone pines are some of the oldest living organisms in the world. Along the Spectra Point/Ramparts Overlook Trail, you'll pass the oldest such bristlecone pine tree in the park, which dates back more than 1,500 years.Bryce Canyon National ParkAn otherworldly expanse of hoodoos and arches make up the geology of Bryce Canyon National Park. Easy access to Sunrise Point, Sunset Point and Inspiration Point allow visitors to watch the shadows creep over the land at sunrise or sunset. Hiking is a must in Bryce, and you need to get below the rim to experience the wonder and awe among the hoodoos. There are many paths leading into the network of formations, and one of the most popular and scenic combines the Navajo and Queen's Garden trails for one of the most spectacular 3-mile hikes in the world. After passing the entrance sign to Bryce Canyon National Park, before you reach the entrance toll booth, make sure to take the road to the left, which leads to the Fairyland Canyon section of Bryce. You'll find spectacular views of the orange-tinged spires without the crowds (or the park fee if you're planning a trip after Aug. 28).[See: 12 State Parks That Should Be on Your Bucket List.]North Rim of Grand Canyon National ParkOver 1,800 million years of geologic history stand before you at the strata of the Grand Canyon. The North Rim is less accessible than the South Rim and 1,000 feet higher in elevation but worth visiting. You can take your pick from hiking to Bright Angel Point for a spectacular view of the Grand Canyon, or sit in an Adirondack-style chair on the back porch of the Grand Canyon Lodge and partake in one of the ranger-led programs. Mule rides into the canyon date back to the 1920s and range from an hour to a half day. Prices start at $45 per person for a one-hour ride to $90 for a half-day trip. For a meal to remember, dine in the Grand Canyon North Rim Lodge's main dining room, which affords unparalleled park views. Just remember to make reservations ahead of time online as spots tend to fill up quickly.While the Union Pacific Railway may not lead Grand Circle Tour trips anymore, an RV makes for a great family-friendly option, even while tooling down the highway. For more information about how to plan an RV road trip on your own, visit GoRVing.com to learn how to get started, where to go, what to bring and more.