Unfortunately, there isn't much to choose from for Thai around Union Sq. There is a fast food version called King of Thai, but if you're looking for any kind of decent decor, that would not work. The closest decent Thai food I've found near there would be Basil, which is on Folsom between 7th & 8th Streets. You could walk it in about 15 mins or grab a quick cab ride. As an alternative, there is a nice (pricey) Vietnamese restaurant near Union Sq called Le Colonial. You can also eat in the bar area, which has nice tables & a nice lounge to enjoy small plates. It is on Cosmo Place, which is between Post & Taylor.