Ok first of all screw Westheimer, and no offense to anyone who enjoys the night life there, I'm just rather biased towards downtown due to my poor experience at westheimer. If you're in Houston then any of the Pappa's based restaraunts are a must. See that Lupe Tortilla over there? Or that Cafe Adobe? They may look nice, but if you want good food then you take yourself over to Pappasitos. South of you on the other side of 610 going West. Look for it next to a car dealership. And this is assuming you are on Kirby. You'll probably have to take the feader to Main and then take the u-turn under 610.
Next up: Downtown. Be sure to hit up Boaka Bar/Mercury Room. These are technically the same club with Boaka Bar being upstairs and Mercury Room downstairs. Mercury Room was voted one of the top five clubs in the nation by Playboy a couple of years ago and their reputation shows. Just try mercuryroom.com and click on the Mercury Room side. This is the place where they had the red carpet out for stars during the superbowl (Pepsi's Superbowl Star Party Smah or whatever), then they went straight downstairs. This is also the place that P. Diddy chose to showcase Danity Kane on his show, whatever the hell it's called. They also have Playboy Bunny tryouts there occassionally. As for downtown clubs, the rest of the ones I used to go to shut down or changed names. Just take the Metro Rail straight until you hit Lousiana I believe. You should end up in the central part of the night district. If you like Japanese food, there's also a restaraunt downtown called Sake Lounge right next to the Verizon Wireless Theatre. This part is called the Bayou Place I believe. Hard Rock Cafe is also located there.
For fast before the club dining downtown, look for a big ass neon sign that says "Cabos" and then right next to it there's a pizza place called "Frank's." They stay open until 2 so you could probably go there after the club, too. Not sure if they beat out New York Pizzeria for best in town, but I could be wrong.
New York Pizzeria (or the one closest to you anyways) is on Kirby and Holcombe. From where you are (and again, assuming you are staying at the hotel across from Reliant on Kirby) it will be going North on Kirby away from 610. You'll pass up Old Spanish Trail, Main, and South Poast Oak to get to Holcombe. New York Pizzeria will be to your right on the corner of Kirby and Holcombe after the stoplight.
Let's assume you for some reason end up on Westheimer. Be sure to go to Chacho's which is on Westheimer near Fountainview going West away from the Galleria. This is open 24 hours, the food is great, and the portions are humongous. If you go there past 10 on a Friday or Saturday you will have GREAT difficulty finding parking. It's almost a club unto itself since all the clubbing people get drunk there then go clubbing or go clubbing then go to Chacho's to get even more drunk.
Hope some of this helps. Enjoy your stay in Houston.