We recently visited with similar desires.
Have to second Chris Madrids as hole in the wall with good burger. Not sure the tostada burger is that much different than cheese burger, but still good. Great atmosphere.
Hot donuts (Original Donut Shop) was also cool. Good breakfast tacos, but wish I had done one of the breakfast platters.
2 Bros BBQ was also very good. And location is very different so would think it might qualify as hole in the wall. Smoked Jap poppers were freaking great.
We are looking to have a reception during the day for a group of 10 people, and are looking for a place with good grub and a "private" room. Does anyone have any suggestions? Looking to stay within 20 minutes of the Galleria area.