Inside the castle is a museum with information about the life of Toyotomi Hideyoshi, the history of the castle and the city of Osaka.
Osaka Castle has a nice park, where locals go to picnic. The area is at its best during the Cherry blossom season, in late March - early April.
You can get to Osaka Castle by the JR Loop Line (Osakajokoen Station), or by Subway (Morinomiya Station).