Countries like Slovenia?

As Europe is a very large continent there are lots of similare countries. I know Slowenia very well but I think there are really more interesting ones.
For beeing able to can better decide maybe first read a few guide-bookes about several countries, like Austria, Slowakei, Poland, Hungary, Croatia or norther Italy.

I thought Slovenia was quite similar to neighboring Austria, except that it’s Slavic instead of Germanic, and Slovenia has no Vienna; although it does have a short seacoast which Austria doesn’t, to compensate. But they’re both clean and tidy, Alpine with lakes, prosperous, with hearty cuisine and good beer and wine, and Ljubljana reminded me of Salzburg. So that’s my recommendation.

You did well with Slovenia!

The rest of the Balkan countries have their own share of terrific natural areas and engaging cities. Croatia, Bosnia, Montenegro, Albania are all worth considering, and I think on par with Slovenia’s charm and hospitality (if not more so.) For example, Croatia has some famous parks like Plitvice and Krka but that whole area was a beautiful and seeming underutilized nature area. Outside wonderful Sarajevo in Bosnia, the stunning Neretva River plays host to rafting excursions and all kinds of pleasure boating. Just to say, there’s more than meets the eye to Slovenia’s neighbors. Check the website for FunkyTours (a local, and friendly tour company) and see if anything jumps out at you.

You can do the same thing in larger countries, just focus on regions rather than the entirety. For example look at the Basque region of Spain, or the Peloponnese in Greece, or Marche in Italy, as just a few examples of places I have enjoyed with a nature and small town emphasis.

Go to Georgia! There is a fantastic blog called wander-lush.org that is a fabulous resource. It's easily one of my favorite countries I've been to.

You might love Scotland. Glasgow and/or Edinburgh offer lots of “city” things, but then there’s lots of hiking/climbing/exploring to do outside of them. Actually, right in Edinburgh, there’s Arthur’s Seat, a climbable extinct volcano that rises over 800 feet (250 meters). I went up there our first trip, and got caught in a rainstorm coming down. Still, worth it! There are also islands right off the coast of mainland Scotland that could offer what you seek, including the Isle of Skye to the west, and Orkney to the north.

While you’re in the UK, there’s also Wales. This year, we’re going there for the first time, so I don’t have personal stories yet to relate, but we anticipate more urban Welsh experiences in the south, and outdoor Welsh opportunities in the north.

Depending on where you’re coming from, in either case you might be first arriving in London, which offers exceptional “City Things” like nowhere else, although it’s also full of parks, and bordered by adjacent hiking/exploring options.

I was last in Slovenia in 2003 so I'm sure it has changed a lot but back then locals thanked you for wanting to see their country.

Five years later I was in Slovakia in the high tatras and it was a similar vibe. This was winter and beautiful.

Vienna is an amazing city but without the same warmth that a smaller town or city has. For more authentic cultural experience a small town will deliver similar to the Slovenia feel probably in a lot of places in Europe, also cheaper.

I would also say try a region--maybe something like the Lake district or Peak districts in England, maybe Dresden / Saxon Switzerland, Alpine France (Grenoble / Albertville / Aix-les-Bains) . . . . Those can all be quite regional, pretty nature, quaint, clean, and with tons of options to get all different kinds of experiences. Wales and Scotland would also allow for some "city" things but lots of hiking/exploring. Each offers the option to go further afield to get more "big city" (it's not far to Berlin from Dresden, to London from the Lakes, or Geneva or Turin from Grenoble). Alternatively, you could choose a city, then look into what nature is in the area (for example, choose Berlin and then look into Saxon Switzerland, the Harz, and the Baltic Coast regions).

Yes, it will be different--the vibe may be similar, but it may not be. That depends a lot on you and what your body feels. And that's great! It WILL be a completely different experience because no place is also Slovenia, no two trips are alike, and even two people on the same trip will have different perceptions. And that is part of the fun.

So absolutely be open to a completely different experience, but still plan the trip that you will likely love--with small vibes, options for exploring, nature, and hiking--while still having the opportunity to tack on one or two city days to see how you like it. Though if you choose Berlin or London, you might find that you enjoy the city sites and want to do more--two days really sells each city very, very short.

Basically, be open, but still choose the options that most appeal to your preferred activities and pace of travel. You'll find there are many ways to have a "trip of a lifetime".

I also fell in love with Slovenia and after reading your post, the first country that came to mind was Ireland, as having a similar vibe. Someone else mentioned Scotland and Wales and I would agree with that. Croatia too, which is very much like Slovenia.