I found the karst mountains in lao very impressive - floating down the river next to these majestic stone formations was super cool. Also exploring some caves near vang vieng. Quite a bit scary since they don't care if you die or live
The countryside in Lao must be very beautiful but moving around freely isn't possible because of the bombs. taking the old road from luang prabang back to vientinne was breathtaking.
But then i always got the feeling that you are unwanted in lao.
A ton of people i met loved to take a motorbike and drive through vietnam.
The countryside in northern thailand is also quite beautiful even though i only know it from the train and daytrips. Some people swear by some kind of hippie village called pai - not longer a secret but have not been there.
Wishing you a good time