South Bellingham has the best ramps going north. The on-ramp is curved but you'll get picked up FAST.
Toward Everett and Seattle, there are 2 on-ramps from downtown Bellingham. There's one on Lakeway or Lakeside or something... and it sucks, don't take that ramp. Take the ramp on King St., there's much more room for people to pull over. There's virtually nobody on Lakeview. But you can still try it if you think you're tough.
Sunset Drive is a state highway running east and west, but locals told me not to take it and that it leads to nowhere, the only point of taking it would be to get to highway 2, which is easily accessed by going south to Everett and taking it from there.
Accommodation and Sleep
There is one mission, The Lighthouse Mission, but there's a lot of homeless people and limited mats, so don't go unless you have to. There are rumours that they got real political and just passed this ordinance saying that between 21:00 and 06:00, you can sleep anywhere except in front of open businesses, but it's local chatter. That being said, passing out in Bellingham you are unlikely to be bothered. If you're a hitchhiker with no real money, you'll fit right in here and find an empowering community of fellow travellers and a lot of local road kids.