Early September is the sweet spot for the Pacific Northwest. The summer crowds have cleared out, the coastal fog usually burns off by afternoon, and you can actually find a spot to park near the Hoh Rain Forest or Hurricane Ridge. Mornings will be crisp, so pack layers, but you get crisp air and golden light over the mountains instead of the July traffic jams.
A typical trip runs about $1400 once you factor in a rental car, gas for the loop, campsites or basic hotels, and gear. Spreading that cost out over time makes it easier to swallow, but you need a solid plan so you are not scrambling when the bills come due.
Remember that the 0% intro window is a hard deadline, not free money. If you still have a balance left when those 18 months are up, interest kicks in on the remaining amount.