In the former grounds of the 18th Century Wang Na Palace, The Bangkok National Museum houses the largest collection of Thai art and artifacts in the country. It's definitely worth a visit, especially if visiting nearby Wat Phra Kaew or the Grand Palace.
Opened by King Rama V to exhibit the antiques and gifts bestowed to him by his father, it once held a reputation for being an ill-organised gathering of dusty relics. That has now changed, with exhibits now arranged into three areas consistent with Thai history, and good English-language descriptions available.