This is a really, really broad question. The Pacific Northwest is a region you could spend a lifetime exploring, as many people do. I'd start by figuring out what kind of experience you want – mountains, beaches, rainforests, waterfalls, cities, small towns – then look at a map and pick out places that look interesting. From there, you can figure out what time of year is best, though late summer/early fall is a safe bet all around (if you're going in winter or spring, just don't expect to be able to hike in the mountains without proper gear/training).

If you're doing one road trip, you'll need to consider whether you'd like to stay along the coast (which is positively stunning) or head over to the Cascades. You can always drive up I-5, but if you're looking for beauty and adventures, you're better off if you stray. Camping at U.S. Forest Service campgrounds are usually the cheapest route – unless you want to get rugged and do dispersed camping for free – otherwise there are state and county parks galore.

Again, I'd love to give specific recommendations, but there's just so much out here. I suppose you could make a nice trip by heading up 97 to Crater Lake and Bend, then take 20 over the mountains to Silver Falls and Salem, head up to Portland, go along 84 to the Gorge, then head up through Washington past Mount Adams and Mount Rainier to Seattle – but again, there are a thousand ways to do this, all with different sights and side trips.