Flying domestically
 

What are the ID requirements for flying domestically in Japan? Can one get away with just a driver's license or a Gaijin card? I hate carrying my passport when I don't have too.

Why not make some effort and check yourself with airlines/on internet. This is a standard question and the requirements are official, not like someone seeking advice on hotels in a particular part of Tokyo at a particular budget when they have never set foot outside the USA, whereas you according to your profile are based in Japan at least some of the time.

ANA below for example.

https://www.ana.co.jp/en/jp/book-pla...de/public.html

Above is FOC.

AFAIK, there's no requirement for a passport. IIRC, I have flown on JAL and ANA domestically with no ID check at all, and as far as I can tell that's still the case. Individual airlines ID requirements may differ (e.g. low cost carriers do check ID, I hear). But I would expect all to be satisfied with a Japanese drivers license or other photo ID issued by Japanese government.

With ANA's "Skip" service, I can skip the check-in counter altogether and can use my mobile app or Super Flyers credit card to get through security and boarding at the gate. No ID check at all.

I have a couple of domestic flights coming up during the national holiday period. Curious what to expect in terms of crowds and allowing extra time before flight to make sure we have no difficulties. We are flying out of HND to HIJ midday on Thu 5/2, and from ITM to NRT afternoon on 5/7. How much time in advance should we plan to arrive?

May 7 afternoon will not be busy at ITM, as the holiday is over and business travelers tend to fly in the morning. Going to NRT, I assume that you're connecting to an international flight there. I'd recommend arriving an hour ahead of time at ITM, though 45 minutes would certainly be fine. You will see special check-in counters for passengers transferring to international fights in Tokyo. Signs are in English.