Catching a sporting event when you travel is a great way to experience the local community and be inside an amazing piece of architecture. Even if you don't care for baseball, it's well worth the price of admission (usually not much more than $25 to nearly any stadium for most games). I've always tried to catch games when I travel and have been fortunate to see professional MLB games played in nearly 30 stadiums, some of which are no long standing. Here's the list, as of Spring 2018.
Arizona Diamondbacks - Chase Field
Baltimore Orioles - Camden Yards
Boston Red Sox - Fenway Park
Chicago Cubs - Wrigley Field
Chicago White Sox - US Cellular Park
Cincinnati Reds - Great American Ball Park
Colorado Rockies - Coors Field
Detroit Tigers - Tiger Stadium (kaput)
Houston Astros - Minute Maid Park
Kansas City Royals - Kauffman Stadium
Los Angeles Angels - Angels Stadium
Los Angeles Dodgers - Dodger Stadium
Los Angeles Dodgers - L.A. Coliseum (old)
Montreal Expos - Olympic Stadium (kaput)
New York Yankees - Yankee Stadium (old, kaput)
New York Yankees - Yankee Stadium (new)
New York Mets - Shea Stadium (kaput)
Oakland Athletics - Coliseum (should be kaput)
Philadelphia Phillies - Veterans Stadium (kaput)
Pittsburgh Pirates - PNC Park
Pittsburgh Pirates - Three Rivers Stadium (kaput)
San Diego Padres - Petco Park
San Francisco Giants - AT&T Park
Seattle Mariners - Safeco Field
St. Louis Cardinals - Busch Stadium (old, kaput)
Tampa Bay Rays - Tropicana Field
Texas Rangers - Globe Life Park at Arlington
Washington Nationals - Nationals Park