I can only think of one hotel that fits the description...from when I worked in travel, guests are probably (but ask if you get the chance, because it's important):
1) corporate sponsors plus invited guests - the sponsor is basically buying the guests' time by providing the cultural bait unobtainable elsewhere - this is your most sophisticated audience, and the sponsors will expect you to be urbane and reliable as well as talented. You may not get this kind of chance, as sponsors will look for a known name.
2)rising businessmen on training seminars - they don't have clients with them, so they are in a bit more of an upbeat rah-rah mood than 1).
3) overseas visitors, mostly couples, from 30s on up. They will expect and appreciate the extra mile from you, but they generally won't want to hear lots of detail from you, just a nod, a smile, and the music.
4) young couples. Probably best to fit the music to the venue (after all, that's the mood they've come for), than specifically to their ages.
For the hoteliers, you are not only a musician, but also part of the decor, and part of the staff - they want to be convinced that if somebody asked you for an ashtray, or made inappropriate comments or impossible requests, you could deal with the matter without major interruptions, or making the guest feel as stupid as s/he deserves to!
As a young man, you already have an advantage in the decor department, so make sure you have a photo or two in your portfolio showing that you can dress appropriately - contemporary downbeat elegant, escaped-from-my-own-wedding, etc. etc.
If you get the job, you may find that they are willing to keep a small stock of your business cards for enquiries.
Musically, I dunno, but if you have experience in accompanying singers or instrumentalists, make sure it's written clearly in your portfolio. If you sightread a treat, or can pick up tunes by ear and arrange on the fly, put that in your portfolio too.
Jazz is evergreen in Japan, but "Latin" piano has been hot here for a while too - malambo, salsa, merengue, tango, or even Cuban...
For quiet, how about something modern and open-sounding like Brian Eno?
P.S. I strongly recommend that you ask if you can audition with a prepared piano piece....

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Good luck!