Visit Yellowstone National Park in June to see bison calves in Lamar Valley, view Yellowstone Falls, and admire the Grand Prismatic Spring and Old Faithful.
In Yosemite National Park, waterfalls are in full flow and many trails are likely to be open. Yes, Yosemite Valley will be crowded, so start your days early.
Great Smoky Mountains is the most visited national park in the US. In June, you'll enjoy wildflowers and waterfalls in full flow, without the crowds of high summer and fall.
June is a great month to visit Sequoia National Park (and nearby Kings Canyon) to admire the largest trees on the planet. Waterfalls in the parks are at their best in June as well.
The weather may be unpredictable, but you'll beat the crowds if you visit Rocky Mountain NP in June. Trail Ridge Road is open, and you'll see wildflowers!