All depends on what part of town you're staying in but I thoroughly enjoy Tony's and also El Tiempo.
http://tonysmexrestaurant.com/locations/
http://www.eltiempocantina.com/
But there are so many good Mexican spots in Houston it's hard to go wrong with finding a good one.
As far as Karaoke goes I've been known to drop a few bars at Howl at the Moon and R Bar once or twice LOL.