Bangkok's Philatelic Museum covers the history of Thai stamps and the postal service, from the first issues under King Rama V to later designs and international examples. The displays include postal boxes from different periods and a room arranged like an older post office.
It is a niche museum, but a good one if you enjoy graphic design, postal history, or small collections with a clear focus. It also makes an easy add-on if you are already near Chatuchak or Bang Sue.