Constructed as residence for the Kings of Siam (now Thailand) in 1782, Bangkok's modern day Grand Palace is one of the most architecturally impressive attractions in the country. Its ornate grounds are composed of an outer court that contains, among other things, Wat Phra Kaew (known in English as Temple of the Emerald Buddha). Here you'll find the small but profoundly revered emerald statue of the Buddha, which is itself something of an aged world traveler.
Though some of the throne halls and residential structures within the middle court, and all of the inner court, are unavailable to visitors, there are still an abundance of beautiful buildings to marvel at. The Abhisek Dusit Throne Hall, for example, expertly blends architectural influences from both the East and European West. At the Amarindra Winitchai Hall, which is still used for certain state ceremonies, visitors are treated to the "old" throne room in all its regal glory.
As a note of cultural respect, it's good to know that cameras are welcome but revealing clothing is not. This is one of the most sacred places in Thailand, after all, so even on hot days it's good to carry a pair of jeans to throw on just in case.