If are you flying home via LAX, hang on to your I-94W (or whatever piece of paper they give you) - tell the airline staff who may want to take it from you that you will be re-entering the U.S. That form is valid for re-entry into the U.S. for the term of the stay (bear in mind the clock for your stay in the U.S. remains on even if you are transiting for a couple of hours to Mexico or Canada due to the contiguous territory rule of VWP.) Don't surrender it until the final point at which you leave the U.S. on your way home.

As long as it is not a Friday, I don't think the volume of northbound traffic in September will be too long. Getting out of Everett at rush hour is what can be a bear. As it appears there will be at least two of you, head for the left to get into the HOV lanes. Even though they end in a few miles, you avoid the traffic turning off for WA20 (or is it WA2?)

Pretty easy to do this w/o a GPS. To get onto 99S, just head south on Howe Street (intersects with Georgia) which joins onto Granville Street (works when driving east bound). Head down Granville Street until you get to 70th where you turn left, and then go a few blocks before heading south (right) on Oak which will take you to the border and onto I-5.

On your way up, take the Marine Drive West exit after the bridge, stay in your lane as you past the Coast hotel after the cloverleaf (left brings you to the airport and right takes you onto 73rd Avenue that you don't want to go on). Just head up Granville and veer onto Seymour as you cross the bridge, and turn left on Georgia.

There are other alternative routes (thanks to the grid system) but I hope this one is the easiest for you.