Totally get wanting to dump bags and go soak, especially with a bad arm. Unfortunately Hawaiian is usually pretty strict internationally. In most cases, they open check in about 3 hours before departure at HND, sometimes 4 if it’s a busy day. Seven hours early is almost certainly too early to check bags with HA. They usually won’t accept them way ahead of time like some Asian carriers do.
Haneda does have luggage storage services in the terminal and they’re fairly well located, but around NYE they can fill up fast. Still probably your best bet if you want to enjoy the onsen without dragging bags around. The airport onsen itself is great and very chill compared to being out in the city during holiday crowds.
If lockers are full, there are staffed baggage storage counters that are easier with a larger suitcase and less walking. I’d aim to store bags, do the onsen, then check in once counters open.
Between Tokyo Station and Haneda, relaxing options are limited without crowds this time of year. Honestly, staying airside as much as possible is the least stressful move. Haneda is one of the few airports where spending extra time actually feels pleasant.
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