Monkey's Kitchen is run by a caterer who's won awards for all the crazy weird fusion dishes she concocts. And, unlike many fusion restaurants, she actually combines Thai and Western ingredients in appetizing ways (the baked mussels with penang curry and cheese are interesting -- in a good way).
More than that, they're very good at presentation here and you get a nice judge-on-a-cooking-show vibe. The astroturf carpeting and stuffed monkey decor is, uh, unique, and there's a big window into the kitchen where you can watch them work. The place isn't really big enough to go with a group of more than three or four people but it makes a great date spot.
The tiny streets behind CMU are pretty difficult to navigate and Monkey's Kitchen is easy to drive right by. Turn left at the second soi on Suthep past the canal, make the first right and look for a little complex with a few cafes. The sign for Monkey's Kitchen itself is quite small.