Acadia National Park
Maine
June - August: Mild Temperatures, fresh ocean air.
October: Blazing fall colors
Glacier National Park
Montana
June - July: Scenic Drives (Only open in summer) and hikes.
September - October: Fewer Crowds; aspen, larch, and cottonwood trees turning yellow.
Great Smoky Mountains National Park
North Carolina and Tennessee
April - May: Lavish wildflowers, blooming redbud and dogwood trees
September - October: Brilliant fall colors that change with elevation.
Grand Canyon National Park
Arizona
April - May: Not as hot.
October - November: Less crowded hiking trails.
Joshua Tree National Park
California
March - May: Cooler weather, cactus blooms, scenic wildflowers.
October - November: Cooler Weather
Rocky Mountains National Park
Colorado
September - October: Elk bugling season; eagle, hawk, and falcon spotting.
Yellowstone National Park
Idaho, Montana, and Wyoming.
May - June: Wildlife Watching (bison, wolves, bighorn sheep, elk.), wildflower season.
October: Cooler weather, fall colors, migrating wildlife.
Yosemite National Park
California
May - June: Yosemite Falls at its peak, migrating birds.
Zion National Park
Utah
October - November: Less crowded, bighorn sheep, fall colors (orange aspens, cottonwoods)