Countries that interest me are: Nicaragua, Guatemala, Sri Lanka, Montenegro, New Zealand. Just trying to decide which to go to!
I've done already: Thailand (6 months), Cambodia (3 weeks), Vietnam (1 month), Laos (1 month), Singapore (1 week), Philippines (3 weeks). I've travelled a lot with family in many other countries (just over 30 countries now) as I was lucky enough to live in Asia and Africa when younger.
Ideally I'd like to go somewhere that's cheap for the total cost (flight and travelling money), warmish weather (although doesn't have to be so hot i'm sweating all the time), sporty and adventurous, plenty of things to do (i hate sitting in places bored), close together (i hate spending my time backpacking on long journeys all the time), something different and out of the ordinary from home, health (i put a lot of focus into healthy habits).
Wow - I recommend the Carribean. Beautiful islands, good food, good music + culture - though I’m slightly biased. Have a look for yourself anyway.
South america? Heading into their summer season now too?
Can't recommend Costa Rica enough.
Maybe not right now (it's the rainy season), but certainly from late December through to July.
Walk across northern Spain on the Camino de Santiago? Various roues available. Incredibly social and historical.
Drive around the USA? Could even extend to Mexico.
Of the ones you mentioned, I've been to Sir Lanka and Montenegro. Both are really nice, but for Montenegro I would recommend a rental car.
Other than that, I can also recommend Costa Rica as mentioned by TattyBoJangles. Many national parks, endless activities, nice beaches, safe... And it's possible without speaking Spanish. Most of them speak basic English.
I was gonna suggest Cambodia but noticed you already have it on your list. South Korea is pretty nice.