My restaurant suggestions for Bangkok -
For authentic Thai food, my favorite is Ruen Mallika off of Sukhumvit Road (casual). Lemongrass (opposite from the Emporium shopping center) and Baan Khanitha (both casual) are also popular Thai restaurants. The Spice Market is a safe choice at the Four Seasons Hotel.
For a nice ambiance and good thai food, i also like the Celadon thai restaurant at the Sukothai hotel (smart casual). If you are in the mood to try thai hot pot, check out Coca restaurant (casual) - they have restaurants in many locations but would recommend going to the original one near siam square. Its causal, delicious and inexpensive (its the sauce thats awesome - you can also order a soya-sauce/ lime/ chilli set and add these to the sauce to change it to your liking - also order a fresh crab to dunk in the hot pot, its sweet, soft and delicious). All these restaurants have english menus.
Giusto is a great italian restaurant (smart casual) with modern ambiance and is my favorite too - its nice to grab a pre-dinner drink at Koi (smart casual) which is near by. Zanotti (smart casual) is a nice italian restaurant as well.
I would recommend going to Sirocco Roof Bar Restaurant (smart casual) for at least a drink - its an impressive outdoor bar/ restaurant (so dont go if its raining!) on the 63rd floor of the State Tower - I forget whether the food was good, but great view and atmosphere and worth the trip.