Depending on your budget and the night, you might try Bistro 45. A friend had a lovely birthday dinner there on a weeknight, when we pretty much had the place to ourselves. They also have a patio which they might reserve for 20.
At the lower end of the spectrum, the Il Fornaio in the One Colorado plaza has a private room that accomodates about 20. Food is pretty decent there, I find.
Santorini has a nice patio, but I doubt you'd get it to yourselves. I don't know if they have a private room upstairs in the restaurant. They used to have a separate space downstairs called Rococo, but I don't know if they still do.
Ruth's Chris may have a private room, but I haven't been to that one yet.