I-5 North from seattle

You can even take buses to Bellingham, starting EARLY in the morning, and for about $6.
I might suggest that, if you are gonna pay at all and take the bus to Everett, but if you wanna do it 'authentic' and totally free, hit the nothernmost I-5 spot you can get to with a "50 mi. PLEASE" sign large and legible. That'll move you to Everett or so, where you can then fly that sign again at the next on-ramp.
The buses will deliver you quicker, for minimal cost.
Please do report what you do and how it goes!
 
This has gotten a lot harder the last few years. But it's really not that hard. As long as you say your going somewhere, that's usually good enough. An address is good, even if it's just the address of a friend or a hostel. If you say you don't know where your going and you don't know who your going to see, they will turn you back at the border, but shit can you really blame them.