OK, there's probably not much I can share that would be helpful. The region has many great things, but you have limited options.
I did not go into city of Sasebo (may have driven past it, didn't notice a large port/city there - Sasebo is also the region name, I believe). The landscapes and seacoasts around there (northwest Kyushu) are pretty, often quite dramatic. Lots of islands, too. I'm sure all of it would be scenic. What month are you going to be there? I would expect no cherry blossoms there after April (it's in the south, so any cherry blossoms would bloom early, like maybe March). It's not tropical, but more semi-tropical (warm) but they do get snow in the mountains during winter time. We were there in May, and found lots of flowers (wildflowers and formal, planted gardens) Don't know much about those islands nearby. If there's an observation tower, I'd expect a lovely view.
I'll only add this: Japan went through a period (1960s-70s, I think) where they went crazy for theme parks. There are (or were) lots of theme parks all over the place, many for oddball things (eg, a Hello Kitty theme park - we gave that one a pass, too). Many of them failed financially, some are still going but have seen better days, some have been abandoned altogether or nearly so.
We went to a "Porcelain theme park" in Arita (just a short drive from where you will be). Arita has been famous for fine porcelain pottery for ages, this place was intended to be like a Disneyland for porcelain, with replica/knock-offs of full-scale, ornate European palaces, a Euro town and all sorts of porcelain-related attractions. That park is now essentially abandoned (there's still a restaurant and porcelain gift shop), it was huge, atmospheric and (to me) absolutely fascinating. It's probably not an option without your own car, and I'm sure not everyone's cup of tea. We loved the place and spent a few hours there, pretty much by ourselves. There were just us and a few young Japanese teens shooting some kind of cosplay video with incredible costumes (a perfect setting for that!), occasionally a lost-looking Chinese tourist would wander through and then leave...It was eerie, empty, and surreal.
My point being that at Huis Ten Bosch, you may find a little "faded glory" and a big, once-popular place that now feels a bit past its prime...or it might be sparkling and busy, but with some surprises. Lots of both around Japan.
Have fun.