Osaka Castle sits in a huge park in central Osaka, so plan time for both the castle keep and the grounds: entry to the reconstructed main keep costs about 600 yen and it is open roughly 9:00 to 17:00, with an elevator up to the observation floor if you need accessibility. You will get a good panorama of the city and a compact history museum inside, but expect queues and crowds during cherry blossom season and weekend afternoons, so go early on a weekday if you can. The castle grounds are free and great for a stroll or picnic, and the easiest access is from JR Osakajokoen or Tanimachi 4-chome stations, with plenty of nearby cafes and coin lockers at the stations if you want to travel light.