OK. I will try.

I was hoping some Seattle friends woudl reply.
I went there last summer for vacation, driving from Seattle to Vancouver and back. I remember it is about 160 miles on I-5. Normal travel time is around 3 hours depending waiting time at the custom and traffic. It was eventless going out. Coming back took much longer time mostly due to traffic on I-5 near Seattle downtown. I was told by locals traffic congestion is common on I-5 over there. So plan ahead.
Bring regular traveling document as you would when you go abroad, such as passport or GC.
Please post if you have more specific questions.