Firestorm, heres my breakdown of Osaka.

Sightseeing
Osaka Castle: Unless you're somewhat into japanese history and artifacts the castle museum is actually pretty meh. That said it was too bloody hot for us to wander the whole castle grounds/gardens to appreciate it. The view from the top of the castle is nice. Has english subtitles... where they feel like it.
Kaiyukan (Aquarium): A MUST SEE in my books, if you're a huge proponent for animal rights you may object to seeing 2 whale sharks in a (comparitive to an ocean) small tank. The aquarium is awesome, needs about half a day to fully appreciate. Entry price is worth it. Has english subtitles.
Peace Museum: Don't bother unless you're into WWII history. Will probably take 1 hour of your time to cycle through. Has english subtitles.
Tennoji Zoo: You don't need to be a PETA freak to be disgusted by most of the enclosures in this zoo. Some of the enclosures are really good though and you get to see a very wide variety of animals. Also entry is super cheap! Will take around 2 hours to do the rounds and see everything. Has english subtitles.
Nagai Natural History Museum: Pretty cool if you're interested in the content. Worth the entry fee. Has english subtitles.
Nipponbashi Den Den Town: Traditional asian shopping streets, plus TFs!

General Shopping
Tempozan Marketplace: It's actually just a cool mall. Worth a visit before or after the Aquarium. Theres a heavily ninja/naruto themed shop in there. Theres also an absolutely INSANE shop on the ground floor of this mall selling pop culture stuff.
Shinsaibashi arcade: Really really long shopping arcade. Has everything except toys.
Dotonbori street: Food, cheap stuff, giant crabs, more cheap stuff. Traditional asian street.
Namba Parks: Cool complex just to gawk at. Mostly designer labels + TRU in the basement.


TF shopping
Yodobashi camera, Bic Camera, TRU, any other department store you find: will have the in-season wave of TFs in Japan. When I was there, Animated Blackout and Shockwave were the most exciting stuff around
Herogangu: Located on Nipponbashi St in Den Den town. Lots of TF's. Much boxedG1 Love. A lot of stuff boxed and loose from other lines too. Location is on kurdt_the_goat's map, not that you'd miss it, theres a giant billboard above the shop front. WARNING: Closed on wednesdays. You must go to this place.
Jungle: Located on Nipponbashi St in Den Den town. It's one block north of Herogangu and occupies a corner shop front. A smaller selection of TF's but cheaper than Herogangu. Highly recommended.
Joshin Super Kids Land: Theres a few of these around, one is a general toy store, one is totally dedicated to gundam. From memory they only had in-season TF's with one or two pieces of older stuff (encores, sky lynx)
Zombie: Located around the corner of one of the intersecting streets of Nipponbashi. Specialises in 'foriegn' stuff. You'll see a lot of Hasbro TF's at ridiculous prices (4000 yen Drift!). Great if you're homesick.
The next street running parallel to Nipponbashi St to the east Has a lot of shops selling other pop culture stuff (non-tf sadly). I didn't get to explore it much but I found a shop specialising in Ultraman, numerous hobby stores and a lot of anime goods stores.