Akihabara. I caught the train there one morning, stepped out of the train station and on to the street, and was promptly overwhelmed by the lights, sounds, and colours, and all the young hip people who know what they're doing and where they're going. I wanted to check out the scene but was out of my depth. I walked around for about 1/2hr or 45mins and just felt old and out of place, and then jumped back on the train and went elsewhere.
Back in Australia, I know someone who runs his own 'hip' store at the shopping mall, one of those shops that has the latest/coolest computer games, fantasy/D&D gaming, comics/manga/anime items, geek culture items, superhero-related items. He knows his pop culture. I told him that if him and I happened to visit Tokyo at the same time, I'd love to tag along when he visits Akihabara and other similar places. I know I'd have a ball because he'll know where to go, what to do, who to hang out with.