Vietnam, Japan, or Sri Lanka in August?

Hi all, I will be travelling Indonesia for 2 months this summer but originally planned for 3 months- due to visa I can only do 2 months. This means I can spend another 3 weeks or so somewhere else budget wise. I have 4 options right now and I'm really unsure. As a prefice, I will be solo, I don't drink alcohol/party so nightlife isn't a factor (if anything its an offputting factor lol), I love nature and animals, vegetarian food options, I do like beach/snorkelling but I would have done plenty in Indonesia, and also culture and history. These are my 4 options and the pros/cons I've found right now:

1- Sri Lanka for 3 weeks- it's on the way toward the UK, and there's affordable flights to there and back home. good hikes/safaris- would be very cool, good vegetarian food.not great transport around from what I've heard. Not sure why but thinking about going just isn't super exciting for me in the same way Indonesia is but I'm sure this would change when it's coser to the time/I'm actually there

2-Vietnam for 3 weeks- again cheap flights to there and then home, plus cheap flights around the country. Super interested in Vietnam war and theres loads of history stuff to see/do, and good food. Not ideal weather though in August

3- Japan (again)- I went in february and I would love to go back to some less touristy areas, I really loved it but its more expensive so would only be able to go for 1-2 weeks, and also ridiculously hot and humid in August. Japan would be more chill and less 'backpacking' vibe I guess which might be nice after 2 months

4- Fly straight home after Indonesia and then spend a week somewhere in europe like spain, my least favourite option since If I'm in the region anyways I'd like to go somewhere else in Asia and europe is obviously like 3-4x more expensive for everything.

Anyone been to Sri Lanka/Vietnam/Japan in August? I'm leaning toward Sri Lanka currently but still unsure! thanks