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.
The monk's quarters and all the other buildings except the bot (main chapel) are all made of wood. The main "merit making" halls feature some finely detailed carved panels set in plainly painted clapboard sides. It is all very clean, and rather quiet for someplace in the middle of things. The temple has a school, so if you visit during a weekday, it might be quite a different scene, with young children running about.
While the old monk's quarters and merit-making halls are truly unique, the Bot is not without its own charms. While traditional in general design, the decorative features are rather atypical. There is much less use of gold on the exterior than usual -- almost none in fact. Instead of the usual gold-tinted mirrored tiles, this one features mostly blue ones. The gable ends feature finely detailed Asparas and Naga figures which are much more prominent that generally seen in Bangkok temples. The exterior is newly restored and the interior is currently undergoing restoration (therefore, its closed to visitors).
All-in-all, Wat Suan Plu is well worth a visit.
If you're staying at the Shangri-La, then Wat Suan Plu is almost right outside your door. Just exit the main lobby and walk away from the river towards New Road. If you're staying at any of the other riverside hotels, such as the Oriental, Royal Orchid Sheraton or Peninsula, then you can take advantage of a free boat shuttle from these hotels to the Shangri-La. The temple is also a short walk from the Taksin Bridge Skytrain station, making it handy from most hotels in other areas.