The RTAF Museum is out by Don Muang airport. Lonely Planet's Thailand guide mentions in passing that it's a "world class" aircraft museum - I think they may be understating the case. The museum has a staggering selection of aircraft in several hangars and outside, including many trainers, aircraft ancient to modern, several helicopters, cargo planes, ground-based trainers (including a full hypobaric chamber setup), models, uniforms and regalia, medals, aircraft weaponry ... A couple of the airplanes are the only remaining examples in the world, including a Japanese Tachikawa trainer. Admittance is free. It looked like they got about twenty visitors a day. I gave the discrepancy between the quality of the installation and the number of visitors a lot of thought, and the biggest problem is probably that aircraft aren't the reason people come to Bangkok. They come to shop or to go to the beaches. It doesn't help any that getting there took about two hours from downtown each direction on public transit.
The aircraft shown is a De Havilland Chipmunk, a trainer. Apparently the RTAF liked this fine Canadian product: they flew it from 1949 to 1989. The Chipmunk was a replacement for the Tiger Moth, possibly the best known trainer ever put in the air.