Yosemite National Park's famous waterfalls are likely to be flowing well with spring melt. Yosemite Valley is less crowded, and Hetch Hetchy offers waterfalls as well.
Grand Canyon National Park has milder weather than in the winter, and lower crowds than in the summer. Enjoy the spectacular rim viewpoints in relative quiet!
At Great Smoky Mountains National Park, go on a wildflower walk to chase the hundreds of varieties in the park. Waterfalls are flowing well in April too!
Headed to Hawaii? Haleakala National Park on Maui features a summit district perfect for sunrises and sunsets, and a lower section with the popular Pipiwai Trail.