When most travelers set out to explore a destination's museums, they usually don't stray from the most popular (packed Smithsonians come to mind). But a host of off-the-beaten-path institutions full of historical treasures and rare artifacts are waiting to be discovered by travelers willing to go the extra mile. Whether your passion is art, history or computers, these museums are well worth a visit (and the extra time it requires to get there). Plus, many are located in buildings so beautiful and important that they're registered as National Historic Landmarks. Here are eight unique museums worth adding to your next itinerary the next time you're in New York City, Washington, D.C., Houston or Seattle.See: Best Historic Destinations in the USAImmigration is one of the country's hottest issues, making a visit to this Lower East Side National Historic Site a timely venture. Housed in a 19th-century apartment building that once played host to nearly 7,000 working class immigrants (at the turn of the 19th century, the Lower East Side was the most densely populated place in the world), the Tenement Museum offers a glimpse into the lives of its former inhabitants. Through guided tours of restored apartments and businesses, "resident" talks (played by costumed interpreters), and neighborhood walks, you'll learn how these immigrants lived and ultimately shaped America's identity. Tour prices begin at $25 for adults, and $20 for students, children and seniors 65 and older. Kids 5 and younger are not permitted on most building tours. If you're at all interested in the arts and cultures of Spain, Portugal or Latin America, then a visit to the Hispanic Society of America, a free museum and reference library, is a must. Its 1908 Beaux Arts building (also a National Historic Landmark) boasts fine paintings, decorative arts and artifacts of Spain, Portugal and Latin America from the Middle Ages through the 20th century. It has most of Francisco De Goya 's prints, but the standout is "The Sorolla Room," 14 gorgeous murals depicting Spain's provinces, by Joaquín Sorolla, Spain's most famous living artist until Pablo Picasso emerged. Did you know that America's first museum of modern art is not located in New York City, but in Washington, D.C.? Eight years before the Museum of Modern Art debuted, Duncan and Marjorie Phillips opened their Dupont Circle mansion collection to the public. In fact, the Phillips is full of firsts: He was the first patron and exhibitor of many then-unknown artists like Edward Hopper and Georgia O'Keeffe. The keen-eyed collector believed that art can "enhance and enrich living," and a visit to the Phillips will surely enhance and enrich your Washington visit, even if you only see Renoir's famous "Luncheon of the Boating Party" or the museum's Rothko Room. Admission to the Phillips Collection is included with the ticket price for exhibitions: $12 for adults, $10 for students and seniors 62 years old and over; admission is free for visitors 18 and younger. This northwest Washington mansion is filled with one of the world's best collections of Faberge eggs and other Russian Imperial treasures, 18th-century French furniture, and the spectacular jewelry of its proprietor, Marjorie Merriweather Post. The Post Toasties heiress became one of the world's richest women, and her lavish estate reflects just how much wealth the entrepreneur, collector and philanthropist acquired in her lifetime. Her estate's 13 acres of formal gardens include a pet cemetery, a rose garden and a Russian dacha, among many other "outdoor rooms" she commissioned to complement the mansion's interiors. Admission is a suggested donation and includes access to special exhibitions like "Ingenue to Icon: 70 Years of Fashion from the Collection of Marjorie Merriweather Post," running June 6 through December 31.The country's leading institution dedicated to architecture, the National Building Museum, was modeled after two Roman Renaissance palaces. Famed architect Philip Johnson deemed it "the most astonishing interior space in America." The 19th-century Pension Building's Great Hall still serves one of its original purposes, housing presidential inaugural balls from Grover Cleveland to Barack Obama. Tours of the National Historic Landmark focus on inaugurations, ghosts and temporary exhibits. From July Fourth through Labor Day, the Great Hall becomes a 10,000-square-foot "BEACH," an interactive architectural installation with an ocean of nearly one million recyclable translucent plastic balls. Access is free to the Great Hall and building tours, but exhibition admission fees are $8 for adults and $5 for children, students and seniors.See: 4 Trips You Didn't Know You Could TakeAviation buffs can't miss a visit to Houston's 1940 Air Terminal Museum, situated about 10 miles from downtown Houston and the Museum District in Hobby Airport's original art deco building. Anyone remember Eastern Air Lines? Braniff Airways? WASPs? America's first WASPs, World War II's Women Airforce Service Pilots, were trained in 1942 in what was then the Houston Municipal Airport. The Hobby Airport lobby is adorned with photos of stars who passed through its doors like Elizabeth Taylor and James Dean (who filmed "Giant" in Marfa, Texas), plus Bob Hope and Humphrey Bogart. Admission is $5 for adults and $2 for children 12 and younger.Venture a few miles south of the Space Needle and downtown Seattle to travel half a century back to the dawn of the computer age. The Living Computer Museum doesn't just offer nostalgia for nerds who remember Congo Bongo, Commodore 64s or Radio Shack's "Trash" TRS-80, the 15,000-square-foot space showcases the evolution and growth of computing. Try out loads of vintage computers restored to working condition and collected by Microsoft cofounder, Paul G. Allen. You'll find the museum in the SoDo district of Seattle. Admission is $6 for adults; $5 for children 6 to 17 years old; $4 for seniors 62 and older and for active military.To see one of the world's finest collections of Asian art, travel only a few miles from the heart of Seattle to Volunteer Park, where you'll find the Asian Art Museum. Housed in the 1933 art deco building that originally hosted the Seattle Art Museum, this branch is filled with 7,000 treasures from Japan, China, Korea, Southeast Asia and the Himalayas, among other regions. Here, you'll find traditional masterpieces, along with contemporary Asian art. Admission to the museum is $9 for adults; $6 for seniors 62 and older and the military; $5 for teens and students; free for children 12 and younger. See: 5 Tours Made for WalkingAbout the author: Marsha Dubrow writes about travel and the arts for examiner.com. You can follow her on Twitter @MarshaDubrow, connect with her on Facebook and LinkedIn. She earned an M.F.A. in Writing and Literature at Bennington College, which published her book "Single Blessedness."