Viceroy is a fun lounge in Belltown. They have a bunch of large couches, and on the weekends they have a DJ keeping the place lively. Not really a dancing destintation, Viceroy is a great spot to stop by for a drink or two, but unlikely a place where you'd spend the full evening.
Although it was St Patrick's Day when I visited, the bar was relatively full early in the evening. As the evening pushed on, the crowd thinned out appreciably (1 AM).