Near the big stations there are usually many pokestops. I don’t know why there are always people near ikuta shrine in sanomiya, but it is clear they are all playing Pokemon go.
Another good place is near Okamoto station. Actually it’s more accurate to say between Okamoto and setsumotoyama station. (Hankyu and JR.) I got 3 snorlaxes there within 1 week!
When it’s hot you probably don’t want to play outside. Nishimomiya gardens is a nice mall with many pokestops and a/c! It is near nishimomiya kitguchi station l.
Osaka too is a good place to play.