This is our favorite Thai restaurant in Bangkok. It is localed in a cute little house with antique furniture and a nice garden. The atmosphere is so pleasant and comfortable. The service is typically Thai, many servers to cater to your every need. We usually pay about $25 for a four-course lunch for three people. The food is excellent. The green papaya salad is a must, but everything there is good. Indicate how spicy you want the food. Here, as all over Thailand, the locals love extremely hot dishes. If you say "not too spicy", it may still be pretty hot to you. Baan Kanitha understand about Westerners. They have another location which your concierge could find for you, but I have never been there.
One note: it never fails that if you ordered a diet Coke in a restaurant in Thailand that caters to mostly Thais, it will be past its expiration date and taste awful! Order your diet Coke in a big hotel or in the touristy areas. Thais simply don't drink it.
When you visit, be sure to do it during shopping hours. My favorite store is just a door or two down from the restaurant. It is Rasi Sayam, on the same side of the street as you walk back to Sukhimvit. Owned by a Stanford MBA who settled in Thailand, it stocks fine handicraft, weavings, pottery and antiques. I always pick up something unique there at a reasonable price. True, it is more than you'd pay on the street, but the quality is much better and you don't have to sift through a lot of junk. Everything there is made in Thailand. Enjoy!