^I second the rnj lounge! really yummy chinese food - and if you like crab the deep fried crab is good too!
Not by Union Square but i've been thinking about their dry fried chicken ever since I went a year ago...
http://www.yelp.com/biz/san-tung-ch...id:bQ5WYlPbFIjtCbLgE-Msgw/query:fried chicken
My BF lives in SF and I've been dying to go back to the restaurant ever since my first taste. He likes to take me out to fine dining but this restaurant is just good, simple food. which i would rank over almost all the fine dining he's taken me.
Just in case you ever happen to be around that area...
DRY FRIED CHICKEN!!!
There are lots of breakfast places, high end to inexpensive. One I enjoy is Cafe Mason. It is casual, reasonably priced, and just off Union Square. It is also 24 hours, and near two other 24 hour cafes, in case you want a late night snack sometime.
http://www.cafemason.com/