I am revising a second planned trip to Japan for the fall. We are going to combine it with Taiwan. On our first trip to Japan we visited Tokyo, Nikko, Kanazawa, Noto Peninsula, Matsumoto, Takayama, Kurashiki and Kyoto. For this trip we will revisit Tokyo and then fly to Taiwan. After a couple weeks in Taiwan we will return to Japan for our flight home from Osaka. I will spend the last 2 nights in Osaka, but was hoping to find a destination not far and easily accessible from Osaka to spend a couple days. It could be a smallish town or just go to a nice ryokan for 2 nights. Your suggestions are appreciated.

My first thought was Kinosaki Onsen. It is about 2h 40m on limited express Kounotori from Osaka or Shin Osaka.

Closer would be Hiroshima and Miyajima, 90m on shinkansen from Shin Osaka to Hiroshima.

But those might be more travel than you want Sounds like you fly from Taiwan on Day 1, have one full day, then fly out on Day 3. You could, of course, just enjoy Osaka. Day trip to Himeji or Nara. Maybe overnight in Nara.

Thanks! How many days/time is flexible as I am just planning Taiwan. We do fly home from Osaka. I plan to fly into Osaka from Taiwan and spend 2 nights in Osaka before departure day to the US. I was thinking of after flying into Osaka going someplace about an hour and no more than 2 hours away for 2 nights. Then returning for 2 nights in Osaka before we fly home. In the past I looked at Nara and it never really grabbed me, but maybe I need to look at it again as it makes sense. The other idea is to check into a nice ryokan within an hour or so of Osaka. That option has a lot of appeal to me.
We were in Hiroshima on the last trip, but I see I inadvertently left that off. sorry.

Arima Onsen would be nearby. IDK, it might be worth a 2-night stay (Kinosaki would be).
Here is a page with transportation info:
Transportation | Plan Your Trip | Visit Arima Onsen, Kobe (arima-onsen.com)

There is/was a ferry between Kansai Airport and Kobe. Then rail from the port to Sannomiya. Sannomiya being downtown Kobe. Easy enough to get from KIX to Arima Onsen, but you could stop there for a night, see some sights (like the ropeway) and go on to Arima for the next night.

Should be easy to find a nice ryokan in Nara prefecture, though no onsen I think. Or elsewhere in Kansai area.

Alternate idea, given you have four nights in Japan, is to fly into Fukuoka and spend some time there / northern Kyushu before heading to Osaka for the night before your departure from Japan. Okinawa.

Thank you again! Funny you mention flying into Fukuoka- our original trip was to spend 3 weeks in Kyushu. Then we decided to see Taiwan instead. Its certainly a possiblity.