Mammoth Cave National ParkKentucky 1 of 6
Deep underground in Kentucky sits a vast chamber of caverns and complex mazes known as Mammoth Cave National Park. It is the world's largest known cave system, with more than 400 miles of explored caves and many more miles waiting to be checked out. Though the weather may be scorching outside, the magic of Mammoth Cave National Park is that the temperatures are cool and consistent inside, with the deepest levels maintaining a chilly 54 degrees.
Ranger-led tours inside the caves take place all summer long, but make sure to reserve a spot before you make the trip as spots frequently sell out. If you want to brave the heat, the park also has over 70 miles of nature trails. When you're ready to cool off, take a dip in the Green River (it flows through the park) for some refreshing ground-level entertainment.
Olympic National ParkWashington 2 of 6
Tucked into the Olympic Peninsula of Washington state, Olympic National Park is gigantic, encompassing nearly one million acres. The park represents a number of ecosystems, including rainforests, coastline and snow-capped mountains. While it's the rain that really grabs your attention, we recommend visiting in the summertime to avoid constant downpours. But don't worry about missing all the precipitation. The Hoh Rainforest gets 12 to 14 feet of rain each year and is still a stunning place to visit during the "dry" season.
After exploring the rainforest, head to the beach for another taste of all this park has to offer. At Kalaloch and Ruby Beach, have the kids search for starfish and sand dollars, or scan the skies for nesting colonies of birds like the tufted puffin. Cap off your trip with a visit to Hurricane Ridge for a jaw-dropping view of the Olympic Mountains.
Great Smoky Mountains National ParkTennessee/North Carolina 3 of 6
Spanning two states, Great Smoky Mountains National Park is home to some of the oldest mountains in the world. It's an incredibly popular place and receives the most visitors of any national park. While the stunning mountain views, potential to spot black bears in the wild and plethora of waterfalls make this a great park to visit year-round, it's the synchronous fireflies that make it an extra treat in the summertime.
A natural phenomenon, synchronous fireflies are the only species of firefly in America that can synchronize their flashes with those around them. The result is a nighttime forest that lights up with flashes before suddenly falling dark--a true wonder to behold.
Crater Lake National ParkOregon 4 of 6
Nearly 8,000 years ago, Native Americans watched as the deepest lake in the United States formed when a volcanic eruption caused a mountain to collapse. The indentation from the collapsed peak created Crater Lake, which today is fed only by snow and rain and is considered one of the most pristine lakes on Earth. While Crater Lake is open all year, it's best to visit during the summer when roads are passable, campgrounds are open and trails are clear.
For a fun family adventure, book a boat tour to Wizard Island, a volcanic cinder cone that forms an island on Crater Lake. Afterwards, enjoy the 90 miles of hiking trails available in the park or Rim Drive, a 33-mile road around the lake's perimeter.
Rocky Mountain National ParkColorado 5 of 6
The weather in the Rocky Mountains is extreme any time of year. Thunderstorms can appear out of nowhere during summer, and snow piles up faster than you can say "blizzard" throughout winter. It's because of this extreme weather that Rocky Mountain National Park is best to visit in the summer when roads and trails are clear of snow.
You'll see wildflowers and blue-sky alpine views of the snow-capped mountains. Enjoy over 300 miles of hiking trails in the park or drive Trail Ridge Road, which offers glimpses of the mountains at roadside pullouts and crests at an altitude of 12,000 feet.