The many temples of Bangkok are each unique and inspiring. But while the big temples of Bangkok are highly recommended, don't pass-up smaller neighborhood temples where you have a good chance of learning about Buddhism in daily practice. Early morning is a good time to visit temples: the air is cool, monks busy themselves with morning activities, and the complexes are generally less crowded.
Thai people make regular offerings to temples and monasteries as an act of merit-making. Supporting the Sangha, or monkhood, brings one closer to Buddhist ideals and increases the likelihood of a better life beyond this one. Many shops near temples sell saffron-colored pails filled with everyday supplies such as toothbrushes, soap, and other common necessities, and Thais bring these and other gifts as offerings to Buddhist mendicants as a way of gaining good graces. You may even see an early-morning Pintabat or a column of barefoot monks carrying their bowls on morning begging rounds.
Small monetary contributions (the amount is up to you) are welcome at any temple. Ablutions at a temple involve bowing three times, placing the forehead on the ground at the foot of the Buddha, as well as lighting candles and incense and chanting. Tourists are welcome to participate in any capacity, whether doing the full "do" of bowing and scraping or just lighting a candle for the suffering in this world, and all are welcome at the temple.
Erawan Shrine--Built in 1956 as a way to appease the supposed evil forces that were taking the lives of construction workers and causing other calamities in the early stages of building what is now the Grand Hyatt Erawan, one of Bangkok's top hotels along Sukhumvit Road, the Erawan Shrine is not old but an interesting testament to the faith (or superstition perhaps) in Thai society. It's a simple statue of the four-faced Hindu god Brahma God, named Than Tao Mahaprom, and sits in an unassuming corner of the hotel grounds, but the curative effect of building the monument (construction deaths stopped and the hotel grew very prosperous) meant that the shrine became, and is, one of the most revered spots in the kingdom. The area is crowded with worshippers day and night and always wafting with incense smoke. It is common to see people bowing as they pass by bus, taxi or above by skytrain. Don't miss it on the northwest corner of the Grand Hyatt Erawan property, near the Chit Lom stop on the skytrain.