My husband and I are heading to Japan in mid September, flying into Tokyo or Osaka. Any suggestions or tips. We like a mix of adventure, relaxed exploring, and staying at least 3-4 days in each location. Is 12 days a good amount of time? Thank you!

Make sure to get your rail pass, 100% worth it. Of course do the famous tourist things, but also try some things off the beaten path... September is still baseball season, go see the Tigers in Osaka. Jump on the train and go south, or north to a smaller city. September is really perfect for travel in Japan.

There's never enough time to see Japan! Definitely see a sumo match (there will be some in September) but book in advance from home. Also, I highly recommend renting pocket wifi from Narita. It's AMAZEBALLS!!

Fish market! Make sure you sign up early.

Is the tuna auction at the fish market worth waking up early in the morning for the line up?

I posted this in the last Japan advice post but here's my blog entry on stuff to do in Tokyo (and elsewhere in Japan). http://melissadreamsofsushi.com/?p=8629
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Make a day for Kyoto, its really nice, and if you do get the rail pass it makes it pretty easy to travel around Japan. I had a "home" base in Nagoya and just took the bullet train to a different city each day. it was really fun and you got to see a lot.

Best part of our trip was cycling the Shimanami Kaido! Not difficult and truly breathtaking! Onomichi is a great town to spend time in too.

Hi, I'm from Tokyo. Hope you will have great experiences in my country!!!! At first, you should make a list of what to do, each in Tokyo and Osaka. Here is some example for a plan in Tokyo: If you are disney ppl, we have Tokyo Disney Land (no alcohol) and Tokyo Disney Sea (you can drink!), beside each other, but it takes a day for each. You can also go to Shrines+temples, make food samples, go to anime+electric stuff town, Fish Market, merry-go-round sushi, cross a famous intersection in Shibuya (or feel the craziness of cloud in Shibuya, make sure you go there weekend), Koukyo (Emperor's house. Check their website, they might have a tour inside), Shinjuku Gyoen National Garden, Ninja restaurant, and Sumo (I think they have games in September, but you should check because it has off season.) I'm pretty sure there are more, but you might already been overwhelmed, I don't wanna make you confused too much.haha
And someone already mentioned, but I also recommend you to stop by Kyoto if you have time. Kyoto and Osaka are beside, so you may enjoy both.
They are not my home base, but some recommendations...
Osaka: Dotonbori street, Kaiyuukan (aquarium), Osaka castle+park, Universal Studio Japan (they just opened Harry Potter themed part in the park!!!), Shinsaibashi
Kyoto: Kinkakuji temple, Fushimi Inari Shrine, Kiyomizu dera temple, Maiko experience (You will transform to Maiko, google Maiko if you don't know, and your husband can wear Japanese traditional men's kimono, it's mostly like for women), Ninja experience (transform to Ninja)
And food in Japan is extremely good (I believe... I can't really tell you because I am Japanese.lol but really good) you should try Okonomi-yaki (vegetable pancakes, both in Osaka and Kyoto, they have different "style"), Takoyaki (Octopus fritter in Osaka), Teriyaki burger (any fast-food restaurant even MacDonald's, I'm so sad it's not here in Toronto...), Sushi (needless to say...), Sukiyaki or Shabushabu (dip thin sliced beef/ pork in to boiled water, you might need to take a look how to on Youtube or ask someone), noodles!!!! (yes!!!!), Natto (if you are adventurous, or like to try new thing), food at "family restaurant" ("Family restaurant" is the restaurants that have so many variety of food, taste good and cheap, most of them have self refill drink bar, you go up to the drink machine and refill, as many times as possible, seriously, ppl spend so long time at "Fami-res")
There are still so many kinds of food you can try, but I think those are major ones
Hope some info help you to have ideas!!!!!