You can participate with any free indoor attraction that has enough content for a standalone tour.

Below is a list of suggested attractions in the US and Europe, based on popularity data and visitor numbers as a guide but you can create a tour in any attraction, even if it's not on the list.

USA

National Portrait Gallery (Washington, D.C.)
National Museum of African American History and Culture (Washington, D.C.)
The Smithsonian (Washington, D.C.)
National Air and Space Museum (Washington, D.C.)
Palace of Fine Arts (San Francisco)
National Archives Museum (Washington, D.C.)
Civic Center (San Francisco)
National Gallery of Art (Washington, D.C.)
National Museum of the American Indian (Washington, D.C.)
United States Holocaust Memorial Museum (Washington, D.C.)
Supreme Court of the United States (Washington, D.C.)
Miracle Mile Shops at Planet Hollywood (Las Vegas)
National Museum of American History (Washington, D.C.)
Library of Congress (Washington, D.C.)
Fort Mason (San Francisco)

Europe

British Museum (London, UK)
Liverpool Cathedral (Liverpool, UK)
St Peter's Church (Riga, Latvia)
Hundertwasser House (Vienna, Austria)
Diocletian's Palace (Split, Croatia)
Votive Church (Vienna, Austria)
Madonna dell'Orto Church (Venice, Italy)
Basilica of Macarena (Seville, Spain)
Siena Historic Center (Siena, Italy)
Gates of Diocletian's Palace (Split, Croatia)
Historic Center of Vienna (Vienna, Austria)
Market Hall (Rotterdam, Netherlands)
Strasbourg Cathedral of Notre Dame (Strasbourg, France)
St. Mark's Church (Zagreb, Croatia)