Okay, I have not seen all 30 current MLB parks -- I have yet to visit the new parks in San Diego, Washington, and New York, nor have I attended a games in Los Angeles or Anaheim. So technically, I've been to only 24 of the present 30.

However... in the past I've seen games at old Municipal Stadium (Cleveland), old Busch Stadium (St. Louis), Riverfront Stadium (Cincinnati), Three Rivers Stadium (Pittsburgh), Veterans Stadium (Philadelphia), Shea Stadium (New York), old Yankee Stadium (New York), County Stadium (Milwaukee), Fulton County Stadium (Atlanta), Olympic Stadium (Montreal), Qualcomm Stadium (San Diego), Candlestick Park (San Francisco), Tiger Stadium (Detroit), Memorial Stadium (Baltimore), RFK Stadium (Washington, DC), Arlington Stadium (Arlington, TX), and the Astrodome (Houston). So that's 17 former MLB parks.

Oh, and then there are the games at 205 minor league and independent league ballparks...