Pen Thai, in my opinion, is one of the better Thai places East of Lake Washington, especially in the northeast neighborhoods. The food has always been excellent, and the scale of spiciness ensures that there's a spice level for everyone. If you like it hot, you can get it hot, but you can also get it with minimal spice.
Plus, I think they have the best Phad Ki Mao in the Seattle area. ^_^
The atmosphere is really great, too. It's clean and modern, with brushed copper and wooden beams. Very classy, but still casual. This business is a gem in downtown Bothell. Baskin-Robbins is just down the street, too, in case black rice pudding doesn't do it for you.