Don't go by car! Seattle is famous for it's terrible traffic jams. If you do drive, try to avoid peak hours. Driving around the city centre can be a bit confusing because, although the streets are laid out quite nicely, a lot of them are one way. It can be frustrating sometimes trying to get where you want to go.
Above is a link to trafic monitor on roads clear or most used times. plus once there they have links to more.