I had a sit-down to see what else I could hammer out, and wound up accidentally finishing the puzzle. I confess I don’t quite get the logic of one or two, but they fit, sooo…
In rough order of solving:
7-across デンキ
14-down キャンプ (and considering how much I love Yuru Camp, that should have sprung to mind sooner)
15-across ヤケイ (annoyingly, the fact that Hakodate is famous for it’s night views was mentioned in the Wikipedia article I was reading for research. Didn’t occur to me to check if Nagasaki was too)
20-down ケソウ
8-across ロープウェー
10-down セッケン (Still not entirely sure what “paper soap” is). This automatically makes 4-across クセ and 6-across ツケ, and honestly, I don’t understand either of them.
25-down クチベタ, which makes 19-across ルイベ and 22-across セタイ.
28-across ヤスネ, which makes 27-down ランシャ
And left until last because I can’t really see how it fits the clue at all, 37-down ミネラルウォーター
And so the secret magic word is プラス
And here’s the completed grid:
