Since most of our travel tends to be off season, I've had the luck of being in a MLB town when there's no one there. I've enjoyed walking around the Giants' Park in SF and Coors in Denver. We've done a couple of trips to Camden Yards in the last several years, I think it's a nice park to watch a game, especially in the upper level. Same with Petco(?) in San Diego. I liked the location, walking distance from our hotel on the river.
I really need to get to a few minor league games near here. Norfolk, Richmond, Fredericksburg and Salem, possibly Lynchburg. The first three are within two hours, just need the motivation.