Okay, first can I say, Lane, that this sounds absolutely wonderful! If you need a roommate, please get in touch. 😊
Seriously, it sounds like an absolute dream vacation, and you have just inspired me. I've been thinking about doing something like this for a while, although I just had the idea of spending two to three months in a country (probably England or Germany) and then picking a few bases. That said, I really like the idea of not getting out and doing day trips all over the place, but just absorbing the culture and the environment.
I do have some questions. Do you want the city, town and rural area all to be in the same country, or does it matter? Is public transportation a necessity? I'm assuming it is as I know you usually don't drive when you're traveling, so you would probably want something nearby. Also, have thought of the time periods for your trips?
But to the matters at hand. My first choice would probably be a small town in England or Scotland, just because it would be the easiest in terms of language, and also I think the most relaxing. And when I read your description, It makes me think of towns in North Yorkshire like Helmsley, which kind of sits out in the middle of nowhere, but there's some beautiful places around it. I really liked Helmsley as it was quite lovely, and there are some even smaller villages nearby if you wanted to check those out.
Of course, there's also Italy, which has gardens galore, and you could certainly find a place like Turin or some other wonderful small town in Italy. And I would be remiss if I didn't mention my favorite small town in Germany; Gengenbach, which sits in the the Black Forest and is just an absolute charmer, complete with half-timbered houses, lovely cobblestone streets, and is the epitome of a quaint German village. but it's also relatively unknown except by the Germans, so you won't run into a lot of American tourists, although I don't know if that is a worry to you or not. And it has the advantage of being close to the French border, so if you wanted a little bit of a change, you could go over and see Strasbourg and travel around.
Must go but I'll be back—I look forward to reviewing this thread as it continues.
ETA: Just saw valadelphia's post and agree (at least with Caceres as I am not familiar with the 2nd town). Caceres was really lovely.