We went to Thailand for our honeymoon in 2014; highly recommend! We leveraged airline miles to fly business class which made the long trip to Asia easy (and very cheap).
We spent the first week on Koh Phangan at a beautiful resort in a villa with a private pool overlooking the ocean. This was our splurge part of the trip, but for the level of luxury, it was still way less than what you'd pay for a similar accommodation elsewhere. (Note: Koh Phangan is known for their wild Full Moon parties, but we were on the other side of the island, and it was completely serene - and I think we were even there during the Full Moon.) We then flew up to Chiang Mai and stayed there for 4 or 5 nights, then onward to Bangkok for a couple days before heading home. We made a pit stop in Amsterdam "on the way home" for another 4 nights (again, able to slot that into our airline award itinerary for no extra cost / miles).
I planned everything myself, and everything went smoothly (accommodations, travel, excursions, etc.) Awesome things we did: during our week at the resort (first week there), we spent a lot of time just relaxing in our private pool. In Chiang Mai, we did a wonderful Thai cooking class (it was a group class but we wound up being the only ones scheduled that day - so it was a private lesson!), visited an elephant rescue camp, and went ziplining through the forest. Only had a couple days in Bangkok, but explored what we could. Enjoyed zooming around in tuk-tuks in Chiang Mai and Bangkok.
Food was amazing, people were so nice, and the scenery was beautiful. It was an amazing trip. Would love to go back. If you can swing the longer end of your proposed timeframe (14 days), I'd do that. Asia is a long way to go for less than 2 weeks.
Of the other places you mentioned - I spent a week in Tokyo last November and really enjoyed it (was there for a couple of days for work, then stayed an extra week with a friend who lives there). I would love to go back and spend more time and get to see other parts of the country. Went to Argentina a few years ago, but only to Buenos Aires. I didn't love it, but S. America in general isn't a big draw for me, so I wouldn't put much stock in that opinion :)