If you're planning a trip to the nation's capital, there are far too many places worthy of visiting to make it to during a typical weeklong vacation. Here are some places that are essential to visit during your next trip to the District of Columbia. All of these places are affordable – in many cases, free of charge – and suitable for the whole family. 

International Spy Museum – Located in DC's Penn Quarter, this privately owned museum has more than 600 artifacts on display documenting real-life and fictional espionage. Upon entering the museum, you'll be given a fake identity – you must memorize your new name, birth date and other details – before gaining access to the museum's many exhibits about spies dating back to ancient Greece and Rome.

National Mall – In the heart of downtown DC, this registered national park includes West Potomac Park, Constitution Gardens and the Washington Monument. The National Mall's many attractions include: The National Air and Space Museum, National Museum of African Art, Arthur M. Sackler Gallery, Freer Gallery of Art, Smithsonian Institution Building, Arts and Industries Building, Hirshhorn Museum and Sculpture Garden, National Museum of American History, National Museum of Natural History, National Gallery of Art Sculpture Garden, Ulysses S. Grant Memorial, National Museum of the American Indian, Capitol Reflecting Pool, Peace Monument, James A. Garfield Monument, Smithsonian Carousel, Joseph Henry Statue and the Andrew Jackson Downing Memorial Urn.

The Smithsonian National Zoological Park – Most commonly called the National Zoo, this free attraction has some 2,000 animals and 400 species, more than 30 of which are considered endangered. Animals that call the National Zoo home include elephants, pandas, gorillas, horses, lemurs, lions, mole rats, monkeys, tigers, tree shrews, seals, zebras, armadillos, gazelles and cheetahs. The National Zoo is a fascinating place for both children and adults.

Library of Congress – With more than 21 million catalogued books and 142 million items in total, the United States' national library is a must-see attraction for anyone visiting Washington, DC. Though the library is open to the public, you must obtain a Reader Identification Card to peruse its selection. Getting a card requires a driver's license, state ID or passport; you also must be 16 or older. Only members of Congress, Supreme Court Justices, library staffers and other federal officials are allowed to check items out.

National Air and Space Museum – Visited by more than 6 million people each year, this Smithsonian Institution attraction features the largest collection of aircraft and spacecraft. Fascinating artifacts on display here include the Apollo Lunar Module, Pioneer H, the Mercury Friendship 7 spacecraft, the Spirit of St. Louis, the Rutan Voyager, the Ford Trimotor and Star Trek's original USS Enterprise production model. Whether you're casually interested in space travel and aircraft or an expert, you'll find plenty to capture your attention here.

These are just some of the places that you should make a point to visit during your trip to Washington, DC. Other attractions of interest include Dupont Circle, the Eastern Market, Great Falls Park, Discovery Theater, the Washington Monument and Six Flags America, among many, many others.