MLB stadiums I've visited (current and defunct):

Minute Maid Park/Enron Field - numerous, since the 1990s

Camden Yards - July 2007 and April 2018

O.co Coliseum - April 2012

Fenway Park - July 2016

Angel Stadium of Anaheim - May 2017

Dodger Stadium - May 2017 

Petco Park - May 2017

AT&T Park - May 2017

Safeco Field - May 2017

Chase Field - June 2017

Kauffman Stadium - September 2017

Busch Stadium - September 2017

Coors Field - April 2018

Rogers Centre - April 2018 

Citi Field - April 2018

Citizens Bank Park - April 2018

Wrigley Field - June 2018

Miller Park - June 2018

Guaranteed Rate Field - June 2018

Target Field - June 2018 

PNC Park - July 2018

Nationals Park - July 2018

Comerica Park - May 2019 

Great American Ballpark - May 2019

Progressive Field - May 2019

SunTrust Park - July 2019

Globe Life Field - June 2021

LoanDepot Park - July 2021

Tropicana Field - July 2021

Yankee Stadium - August 2022

Astrodome - numerous games, 1980s/1990s (defunct)

Arlington Stadium - early 1990s (defunct)

Globe Life Park - three games from 2014 to 2019 (defunct)

I was rained out at eight parks but made them up. Three of those (Wrigley, Guaranteed Rate, and Miller) were actually not because of rain at the stadium; that trip was scheduled for August 2017, and I couldn't fly out of Houston because of the arrival of Hurricane Harvey. I was also at the Toronto game in 2018 where the icicle crashed through the roof, but I made that up with a doubleheader the next day. 

The spring and summer of 2018 were a lot of fun, and then I returned to graduate school that fall, which shifted my priorities. I thought I could get to all of the active stadiums by 2020 but I had trips to Florida and NY delayed by COVID.

Other parks I've visited:

I’ve been to around 25 MiLB, independent, NCAA or collegiate summer league venues (mostly in TX but also in TN, LA, GA, NY, CA, OH, MI, WI and IL).