My wife has finally agreed to go on a backpacking trip with me this summer. The problem is that she has chronic migraines and trying to hike a trail with more than 200 or 300 feet of elevation gain per day will trigger a migraine and she'll end up in bed for the rest of the day.

I am looking for trails in the western US (we live in Seattle) that are relativity flat, where the water sources aren't too far apart, and there is great scenery. Any suggestions?

EDIT: Just for clarification, I am not looking necessarily for low elevation trails, just trails with out a lot of vertical elevation gain per day. Migraines, especially with my wife, can be caused by moderate to severe exercise. She seems to do fine on flat trails but climbing up a 10% grade for any significant amount of time would put a very quick end to any trip.