September is the sweet spot for the Pacific Northwest. Summer crowds thin out, the Olympic Peninsula rainforests feel crisp, and Highway 101 clears up enough that driving feels relaxing rather than stressful.
A solid week of car rentals, coastal cabins, ferry crossings, and good meals runs around $3,000 for two people. You do not need to drop that all at once if you plan your payments ahead of time.
Keep in mind that the 15-month intro window is a firm deadline, not free money. If you still carry a balance when the promotional period ends, the standard interest rate applies to whatever is left, which quickly drives up the cost of the trip.