Hi!
So I am stating now, I might have a high tolerance for too kitschy/too much vs. some.
I wanted to try someplace to eat at Nabana no Sato, at least for cake, but my tween was already a little anxious from the crowds. The prices seemed normal for Japan? Again, we were there for the big light-up. There looked like a delightful free foot bath but my tween also rejected this. The paths were nice to walk upon, the light up is really spectacular in places. The tunnels of lights are amazing and the begonia garden was really nice and much more extensive than it looked from the outside. We saw a few trees that still had color as well. It is a decent distance away from the onsen. I got lost when trying to return my rental car to Kuwana and accidentally went around Nabana no Sato. Kuwana felt like it had seen better days. The large building next to the station was closed and I think it was being replaced.
I didn't pay for any extra services at the spa. I thought about doing it in Tohoku later in the trip, I would say... the onsen are less kitschy than Yunessun (either bathing suit or naked.) And especially during the morning, it just felt like some really nice rotemboro with good scenery. I can't honestly say they made me think Oirase (which I have walked in October '09) but they do feel like rotemboro out in nature, at least when the kids are not there. There are also a lot of rotemboro. I would rate them higher than some rotemboro I've actually used in nature, since they were very clean and again, they know what they are doing in terms of creating an attractive environment.
We did eat at the Nagashima Hotel. I was honestly just so relieved to get a reservation I didn't request any specific accommodations and figured we could just eat plain white rice. lol I ended up eating tempura (since I will do same cooking oil) and my tween got kappa maki. I expect if you like seafood the dinner buffet is amazing. I forgot what I got for breakfast, probably yogurt. I know my tween probably had toast. The breakfast buffet was extensive and I've had better but I've also had worse. I was just disappointed in the coffee after my earlier hotels in Japan.
We ate strawberries for lunch. Since I was paying 4800 yen for strawberry picking (the tween insisted on being counted as Jr. High.) I was going to get our money's worth. We ate a LOT of strawberries. They had an interesting exhibit about olive oil, but it wasn't the season. On the 28th we left the area around 10ish in the morning as my tween wanted to get onwards to Tokyo. I wanted to do a little shopping or the theme parks, but my trips are always a compromise.
If you're asking did I like better the onsen at Airinkan actually in Tohoku or Yuami, it's definitely Airinkan, but I enjoyed my overall experience enough to want to go back. The hotel staff were really great. I want to say I paid about 34,000 yen for the hotel which included: free admission both days to Yuami no Shima, dinner, breakfast, parking, free admission to Nabana no Sato both days, free shuttle for the light up, and free admission to the begonia garden for 2 during one visit. I'd booked via JapanIcan, so I had to book as 2 adults, because their website just doesn't handle 1 adult and 1 child of any age at all. (They will show no rooms available, but if I switch to two adults suddenly lots of rooms are available, so I just then need to make sure a location isn't adult only.) So at about 17,000 yen per person, I felt like I got a lot? The room had an actual balcony and overlooked the water, which I thought was really nice, since it was a different view than I normally get.
I think all the webpages in English don't really give a good sense of the area, this is particularly true of the onsen which I thought would be smaller based on the website. I think foreigners tend to only go for the amusement park and not for anything else.
I've not been yet, but since we are going about the same time... I've been looking at
https://e-flowerpark.com/access/in Hamamatsu because it's known for its hydrangea and late June is hydrangea season. I don't know if it is on your list.
I've also been looking up hydrangea viewing sights in Kansai that are not just the usual suspects, though we do plan to re-visit Mimurotoji just because it was fun and I like hydrangea themed shaved ice.
If you are going to Tohoku during hydrangea season, rent a car and go to Michinoku Hydrangea Forest
https://www.ichitabi.jp/spot/data.php?p=36We went on a week day in 2019.
I hope this is helpful!