I've been to all 63 major US national parks. There are 6 I'd recommend to first-time travelers.
Briefly

Rocky Mountain National Park in Colorado provides stunning natural beauty and easy access from Denver, making it ideal for day trips. Timed-entry reservations are necessary during summer months. The park features scenic trails leading to breathtaking lakes and waterfalls, starting from Bear Lake, as well as Trail Ridge Road for spectacular views and wildlife sightings. Arches National Park in Utah is known for its striking red-rock formations and over 2,000 natural sandstone arches. It is easily accessible from nearby Moab, making it a perfect destination for those seeking stunning vistas without challenging hikes.
Rocky Mountain National Park is easy to access for a day trip from Denver, requiring timed-entry reservations in the busy summer months.
Hiking to incredible lakes and waterfalls on easy to moderate trails starts from Bear Lake, while Trail Ridge Road offers sweeping views.
Arches National Park in Utah is visually striking, featuring over 2,000 natural sandstone arches and easy navigability.
Its proximity to Moab and beautiful drives make Arches a perfect introduction to red-rock landscapes without strenuous hiking.
Read at Business Insider
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