I live in the Washington suburbs and think Nationals Park is great. (And I hate to say it, but I think the Nats' chances are much better than the Twins' this year.) If you come up to D.C. for something else on a Saturday when the team's away, I highly recommend the behind-the-scenes tour. You get anecdotes about baseball history in Washington, learn about architectural and environmental features, go through the luxury suites, the pressbox, and the clubhouse, sit in the dugout, AND throw a pitch in the bullpen!
Camden Yards is certainly attractive, not counting the local product on the field. A little-known gem next door is Geppi's Entertainment Museum, which is an amazing collection of pop culture memorabilia. Steve Geppi owns the company that has a virtual monopoly on the distribution of comic books, and the crown jewel of the museum is the comic book room, with his collection of every historic comic you can think of, but there are other rooms devoted to toys and games, figures and dolls, cartoon clips, movie posters, etc. from different periods in history. A must-see for comic fans, and well worth a trip for anyone who wants to say, "Hey, I remember that toy / game / doll ...!"