Wat Ratchanatdaram is a temple less for tourists than for devotees, worshippers or generally those interested in the sacred. It is a working, very well-doing monastery that is more about itself and less about how it looks. It is a bit hidden in the sense of a tree being hidden in the forest, in that it is in the middle of other Rattanakosin attractions but much less spectacular, or loud. I find the Buddha image in the main viharn one of the most stunning and accessible in all Bangkok.
The Loha Prasat is too incredible to describe on tripadvisor - an iron-spired stupa with the inner structure accessible mostly for meditation, and as many people attest, with an immense spiritual power. This is a building to focus in, to think only good thoughts or none at all, and to respect where you are. On top, a tiny chapel open to all sides with relics of the Buddha, to which one ascends up the most perfect wooden spiral staircase.
The Milkey Tree Café behind the Loha Prasat is great for respite; beyond that is an exit that leads you to one of the best-priced, comprehensive, and non-touristy amulet and devotionals markets in the city.