I did a pretty typical tourist route first time, which was ho chi min - Nha trang - hoi an - hue - hanoi - halong bay - back to Hanoi. All with overnight sleeper buses.
I did it all with no pre-booking, and would say that was the easiest. Everything is super easy and cheap to book once you are there.
If you want to be lazy, there are plenty of companies that run tours based on what you are describing, they book your accommodation and transport, then you are free do to what you like when you get to the different places. I've used Intrepid when i visited Laos; the guide was excellent and we didn't have to do any thinking at all, just enjoy the experience.