[samurai007]

When I lived in Japan I visited Osaka about once every 5 or 6 weeks, as it was the closest large city. My train went right into Namba station in the southern part of the city, and I would typically wander around the covered shopping area just outside Namba, and then head out to one or more of the following: American mura, Den Den town, Dotunburi, or take the subway to the shopping area around Osaka station in the north. Osaka castle really isn't one of the best castles Japan has to offer, but you could see that. The Osaka aquarium is nice if you like aquariums. For eating, there are tons of restaurants around Dotunburi, or you can go down to the basement level of Namba station which has a lot of restaurants. (My favorite place down there had huge tonkatsu for a very reasonable price... either pork or chicken. No idea if they are still there or not.)

I don't know what your full itinerary is like, but are you visiting many temples or traditional sites, or do you just prefer bigger cities? A trip to Nikko when you are in Tokyo might be a good idea, and in the Kansai area, Nara or Koya-san may be more interesting and memorable than Osaka, IMO.