Hi I do not live in Japan, but my dad was born and lived there for 12 years while my mom went to college in Japan. I know some knowledge about Japan however... 

Well first of all, you must have a job to be able to live in Japan... Or be married to a native Japanese which changes the situation, but you will definitely be required to work/study in Japan if you are from another country.

Regardless of that, the warmest place is Okinawa and many surrounding Ryukyuan islands (Ishigaki, Miyakojima, Tanega). The Ogasawara island chain is also warmer than mainland Japan, but is very limited in population (under 10K people live amongst all of these islands altogether). Kyushu would probably be a very suitable option for all around year weather. 

As for the countryside, there are many areas all scattered around Japan that have very cute traditional looking towns. If you want hills, then many central prefectures like Gifu, Nagano, Yamanashi and Niigata (inland Niigata) all contain Japan's highest mountains. Northern Japan in the Tohoku region also has many 'hill-like' mountains scattered around, while Hokkaido also has many countryside towns/villages. 

The Chugoku region and even Shikoku have smaller mountains (hill-like) scattered around. However, a lot of countryside areas can be found all throughout mainland Japan besides in the major cities.

@kraut9 Since I don't know much about japan places that i'd actually want to live, and only recently have been getting more info on this it's really difficult because I'd have to choose a place that can sponsor me a visa for teaching english and I feel like they'd only do that in bigger places like osaka and tokyo, and although I'm fine with osaka- Tokyo isnt a place I'd want to live. Fukuoka, however... from what I know it sounds and looks amazing. Plus the people there are really young as japan has a lot of elderly people. Nagoya is another place, but as of now I think my plan would be to just get any job that pays average and is willing to sponsor me a work visa (teaching english), and from there i'd decide where I'd like to end up while I travel Japan over the years.

Is that a bad plan, and seem silly?

For "tropical" you need to go very south, try around Fukuoka. 'Countryside' isn't really a problem in Japan. I went to a few areas, like Nagano (Hakuba), also Hokkaido - it's stunningly beautiful!

哈尔滨

I understand. I plan on getting a visa thru english teaching. Thank you a lot for the response, it's pretty informative. I plan on doing more research based off what you wrote, so thanks again! Appreciate it a lot.