BKK should be perfect for you. You could walk around for hours if you wanted, with lots and lots of long concourses without restrictions. Level 4 is a bit difficult to navigate between all the people and shops and is primarily just Concourse D. You can't walk so far out on the concourses on Level 3, being they have the security checkpoints. But Level 2 is a walker's dream. No security checkpoints and AFAIK you can walk to your heart's content all the way to the end of the concourses. Concourses C,D,E,F, and G are open on Level 2 to international passengers. Level 2 of Concourses A and B are for domestic use, so you cannot get into there, but can see the domestic passengers going to those concourses on the other side of the glass if you pass by there on Concourse D.