I was a bit surprised to see that nobody has covered Zenna yet. To be really brief, it's a hybrid Japanese/Thai restaurant in Plano on the 75 service road. If you head north, exit Parker, and watch on the right you'll see it.
They have happy hour 7 days a week 3pm-7pm where sushi is $2-3.50 for 8 rolls and it's good stuff. Also a full Thai menu. More importantly, during happy hour, draft beers are $0.50, you can get huge beer-towers with 3 liters for $9, and - most importantly - if you are under 21 and looking for a venue to drink at, they often do not card as long as you are not with a huge group and aren't blatantly underage.
If you go regularly it's easy to get to know the hostess and waitress, and if you find a regular waitress who will serve you, it can be a great DHV. "Hey, I know a place we can drink, this Asian chick's got my back". Wells are $2, margaritas $3... It's pretty good, and nice ambiance. If anyone wants to go check it out sometime give me a holler, I'm always down.
Oh, and they're open until 3 AM!