Wat Suan Plu
It's really not all that hidden. Wat Suan Plu is a small temple located on the side street which leads to one of the bigger tourist hotels - the Shangri-La - but it seems to be passed over by most of the tourists who stay there. The temple has also been passed over by the generally unsympathetic modernization that have made many of Bangkok's temples much less attractive than they once were. It remains a relatively authentically Thai-Chinese structure in the middle of modern Bangkok.