Located in Samut Prakan, around 30 km int the South of Bangkok, this open-air museum offers more than 120 replicas of monuments on larger or smaller scales, but often at the actual size of the the most remarkable buildings of Thailand or neighboring countries. There are also some original creations that you will only find here. Muang Boran, it's a little Thailand, a little Laos, Cambodia, China and Myanmar in one place. And I must recognize that the work they did here is remarkable. If the weather is good, the place that you can visit by bicycle (what I recommend), tram (1h30 tour) or golf cart (there is a rental fee) is very photogenic. Even if it remains a little artificial and obviously does not replace the visit of the real monuments, it is nice to stroll in the park by bicycle. Count at least half a day to visite Ancient City as there is a lot to to see. You can eat there, near the floating market for instance.
=Note that if Muang Boran opens until after dark, few monuments are lit so if you hope to make beautiful night pictures like I did on my second visit here, you may be a little disappointed. If the tram ride can be a good introduction, it is quite limited with only 4 stops of 20 minutes on a 1h30 tour and it would be a great pity to limit your time in Muang Boran to this only mode of visit. But to spot the different monuments before taking a bicycle to discover the palace more thoroughly, it may worth it to start with that. Often the visit of Ancient City is combined with Erawan Museum a few kilometers before.