As for the onsen in Arima, it turns out to be a pleasant and interesting experience for us as first time users. Although the hotel is not far away from the train station, it does require a walk uphill so getting the hotel pickup service is strongly recommended. We did as you suggested by asking the tourist office staff to call the hotel for us. Overall, the hotel is quite big but kind of worn out. We had a really big tatami room (which could accomadate 5 people), and surprisingly, were told that our dinner and breakfast would be served in our room. (I thought you told us meals would be served in the dinning room). The food itself is average although the quantity is adequate.
The hotspring area is clean and well-kept. We went to the private hot spring (cave), the open-air one and the indoor one. we enjoyed the privacy of the cave spring but find the open air one more spacious and comfortable. As this is our first hot spring experience, we have nothing to compare with. Overall, we did enjoy our stay. If we were to go again, we would choose a smaller establishment with a garden setting.