Osaka Castle was pretty cool. You can go right to the top of it and see Osaka. There was a baseball field across from it where I spent a good 20 minutes watching a game. Worth it for that alone if you're into observatories/seeing stuff from high places.
No interest in visiting Den Den Town?
http://en.wikipedia.org/wiki/Nipponbashi
I mean part of the one day pass, they have free admission.
And my last night in Japan will be in osaka too, so I'll be visiting that area at night on my last night.