My suggestion: don't say sister and soon to be wife, I assumed it was one and the same person fkr a moment 😭

The other more important recommendation is any national park that is convenient for you. The different ecosystems and environments of the world is what I find special.

Sand dunes, waterfalls, volcanoes, whatever you want.

If you don't wanna walk too much, go for a picnic or smth. Get local street food or the infamous Japanese 7/11.

https://www.japan.travel/national-parks/ Here's some activities too. Have a good time.