Nathong Restaurant
This Thai restaurant is featured in the movie Bangkok Dangerous, where Nicolas Cage takes the deaf girl to dinner. It is not upper class by any means, but the seating area surrounding the pond looks nice. There's a large menu, making it hard to pick... but that's only good!
Somboon Seafood
This place offers good Chinese and Thai-style seafood. Be sure to try the curry crab, one of their specialties.
Ratchadaphisek has plenty of seafood restaurants that cater to high-end Chinese and Japanese tourists though are still quite affordable compared to Western countries. The Huai Khwang area has lots of restaurants, especially good for late night dining. If you happen to be in Royal City Avenue RCA, south of the road is RCA Plaza with plenty of food outlets, a food court and a TOPS Supermarket.
Near MRT Phra Ram 9 lies Ratchada Soi 3 that is home to some excellent street food at low prices. You might want to try some khao man kai chicken and rice with chili sauce or moo yang grilled pork here. It won't cost you more than 80 baht.