Not very many places are open. I had 5 odd hours there in April and it was the lounge or the lounge. Half of the jal first lounge was closed too.
OK, the 10 hours spent there in HND T3. It went surprisingly smoothly.
Only the right-hand side of the first lounge seems to be open continuously. The left side was open yesterday morning before JL6 to New York departed at midday. That pretty much emptied the lounge until late afternoon and the left side was closed from there on. Upstairs was closed all the way through.
Something I would have loved to have in the lounge was a place to lie down for a nap. But the armchairs in the Red Suite work OK for short naps. If you want horizontal, you need to go to the gates and sleep on the benches there (at least now, the area near gates 106-107 seemed pretty much unused and very quiet).
A fun fact: my sport watch recorded about 1.5 km when I did a return walk to the ends of the main terminal area. Given that the watch's GPS reception was nil, it's probably not accurate but rather illustrative. However, the distances shown on the map also support it (the terminal was open from gate 106 to the entrance of 140-149 wing). If you need to go for a walk, a lap or two will give you some distance.
A dietary caution: On my way out on a Saturday evening few weeks ago, only one eatery and one bar was open in the main terminal area. On the way back, this week on a weekday daytime, two eateries and the bar. If you lack lounge access
and have dietary requirements, you'd do wisely to pack some snacks that carry you through the transit / wait time.