Things were way "looser" at a lot of places in the 1970s when I was little. My dad and I were big into sports and stadiums, so we traveled to games and places with stadiums often to see games, but sometimes just to walk in.
JTS (many times as a kid, a college student, and adult)
Metropolitan Stadium (where the Twins/Vikings originally played--went there often to watch games and roam the field--now it's the Mall of America)
Kinnick (same day that a drunk guy fell on my mom, so we never went there again as a family)
Royals Stadium (now Kauffman Stadium, I think)
Colorado
Colorado State
The original Mile High Stadium in Denver
Wrigley
Drake Stadium (never locked)
Have not been able to walk on anywhere recently except for JTS in the summer.