I have fond memories of Shinjuku at night. The first time I visited was in 1997 on an epic trip with fellow exchange students and friends, Damon, Louis and Mike.
There was a bit of a story, but the gist of it is that the place we’d arranged to stay at fell through and we ended up wandering Shinjuku at 2am, hauling our backpacks around while looking for a place that would take us in. Eventually we found a capsule hotel, which was an absolute ton of fun and it all worked out. Honestly, getting yourself into a jam (within limits, of course) is the best way to have truly memorable adventures while traveling.
One night a couple weeks ago while Mary was out of town at a conference, I was exceptionally bored and decided to take an evening walk around Shinjuku.
Now if only I could remember where that capsule hotel was.