South Dakota officials hope a pair of high-profile milestones and stable gas prices bring vacationers to the state during the 2016 tourism season, which will be well underway by the end of May.

The 75th anniversary of the completion of Mount Rushmore National Memorial and the National Park Service's centennial celebration are expected to help fuel interest and bring in visitors, state Tourism Secretary Jim Hagen said. Tourism is one of South Dakota's most important industries.

About 13.7 million visitors to South Dakota spent nearly $3.8 billion in 2015, a strong year for tourism buoyed by the 75th Sturgis Motorcycle Rally and the 50th Custer State Park Buffalo Roundup. The Sturgis rally brought in a record 739,000 visitors, while the buffalo roundup drew more than 40,000 people.

"Sometimes you have those really special years," Hagen said. "All in all, I feel good about 2016 as well."

Mount Rushmore is hosting events to celebrate the anniversary of the carving's completion, said Maureen McGee-Ballinger, chief of interpretation and education at the memorial. In 2015, roughly 3.3 million people visited the faces of four former presidents carved in towering granite, and so far this year visitation is up about 13 percent over the same period last year, she said.

Long tied to Mount Rushmore trips, the famous roadside stop Wall Drug Store is seeing sales growth of more than 7 percent over the same time last year, store Chairman Rick Hustead said. The tourist destination on the route to the Black Hills is bringing in extra help to prepare for more travelers, and Hustead noted that gas prices are "hanging in there."

Mary and Bob Kauffman enjoyed their first two-week trip to Custer State Park a few years ago so much that they're returning Saturday to stay for about a month.

The couple drove from Florida to South Dakota and are set to stay into June, said Mary Kauffman, a 69-year-old retired nurse. They want to take in the wildlife, visit Mount Rushmore and check out Deadwood.

"Florida is great, but it's flat," Kauffman said. "You go someplace like Custer and, I don't know, the scenery is just amazing, and to see Mount Rushmore — it just makes you stop and stare and wonder, 'How in the world could they do that?'"

Memorial Day seems to "open the floodgates" for visitors to the National Music Museum in Vermillion, said Patricia Bornhofen, museum manager of communications. With about 1,200 instruments on its floor and roughly 15,000 in its holdings, the museum has some of the finest string instruments in the world, she said.

"We definitely have a different attitude toward summer," Bornhofen said. "It's the season that everything comes alive, of course."