Americas on Post Oak is great for interesting south american food. I like Simposio, a few miles down Westheimer b/t Voss and Fondren for excellent Italian. There's also a great, small Japanese restaurant called Teppay in the shopping center at Westheimer and Voss - a few miles from the Galleria. Go inside the loop to Montrose / Midtown / Rice Village for a much larger selection of fine dining / less chains.
Thien Ahn on Travis is Midtown has, I believe, the best Pho in town - also killer Bun. While you're in the area - try Ibiza on McGowen. Delicious American cuisine with a Spanish influence, and incredible wine selection. I HIGHLY recommend both Hugos for Mexican and Dolce Vita for Italian, and will throw in Blue Fish on Richmond for some great Sushi. Also, for great mexican I would hit Teotihuacan just north of downtown. Mockingbird Bistro - which is pretty upscale - on Welch is one of my favorites for consistently great French meets American cuisine. For great / interesting seafood try Reef - in Midtown on McGowan - great atmosphere and wonderful food. For BBQ i usually go to either Luling City Market on Richmond, and I can't resist the BBQ sauce and chopped brisket sandwich at Goode Co. on Kirby. Best steakhouse in town is definitely Vic & Anthony's downtown - you won't find a better piece of meat in the city.