It never hurts to look.
Case in point: a few weeks ago I walked out to the post office to mail some packages. I took a slightly different route back than the time before. As expected, I found myself on the wide street heading back towards the train station and where I work. I have traveled on that street many times, but only a few times had I made it that far down the street - actually at the end where it has a T-intersection with a major route towards my neighborhood. On the corner sits a building that I had originally dismissed as simply a pachinko parlor. This summer, I checked out the first floor, which has some nice mall-style restaurants and a store that sells goods from Hokkaido.
But this return trip was different. I went back in and made my way up to the second floor. I found a well-stocked sporting goods store and a couple stores geared to anime fans. The sporting goods store is behind me in the picture above, with a CCG (collectible card game) store to the left and an Animate store directly ahead. CCGs are very popular here in Japan, and I've written about them on my other blog (a better, more detailed post will be coming shortly). And I believe Animate is the biggest anime chain in Japan. The store carries comic (anime) books and plenty of goods based on the books.
There is a very large amount of empty space in the building, as you can see. Japan tends to overbuild and I frequently see poorly used space (goods are really spread out for no logical reason) or abandoned/empty parts of stores (two entire floors at my local department store) in the suburbs. Of course, in Tokyo itself everything gets shoehorned in.
But regardless of where you are, it really helps to look up and explore upper floors. In Akihabara, the higher you go, the weirder the goods are - which is great! Head upstairs at a game store and you could find yourself looking at some great toys, or you could learn all about some really crazy fetish captured on DVDs, such as balloon popping or volleyball girls. Yes, Japanese men lust after volleyball girls. Maybe it's the tight short shorts? If you're in Ueno's shopping district, sometimes better deals on goods can be found on an upper floor. In Shibuya and Harajuku, at least half of the stores can't be entered at ground level.
So when you have a chance, take a few minutes to look upstairs. You might be pleasantly surprised!