I would advise that you get a good paper map and have a good read around the RTA forums and check out the planning pages in the tool bars above. The Map centre is good for plotting potential routes and finding attractions in the areas you will be travelling. Once you have got some dots on the map I'm sure we can help to 'fine tune' and offer suggestions and tweaks.
For me, when visiting National parks there is nothing quite like staying in the heart of the park at night, maximizing your time there and enjoying a night around the campfire. They are quite reasonable to stay in [around $20] and if you are spending 2 or more days there, you can save some money on fuel not having to drive in and out to find a cheap [or free] camping spot. The only problem is is that they do sell out and booking in advance is recommended, but that does cost you some freedom. [The later you are into your trip the higher chance of finding available sites]
Between Seattle and Rocky mountain NP in Colorado you could visit Yellowstone and the Grand Tetons, perhaps even venturing to Glacier NP first. Check out the 'Going to the Sun road in Glacier and the Beartooth Highway in Yellowstoine. While in RMNP you may get the chance, weather permitting, to drive the Trail Ridge road across the park which is also the highest continous paved road in the USA.
From Colorado to the Grand canyon you have many options. Near Colorado Springs are Garden of the Gods, Cave of the Winds and Pikes Peak. You could cut down to Canon City [Royal Gorge bridge] and take US 50 over the continental divide at Monarch pass and through Currecanti Nat Rec area to Black canyon of the Gunnison. At Montrose you could head south on the spectacular 'Million dollar Highway [US550] through the mountain towns of Ouray and Silverton to Durango. Between Durango and Cortez you will find Mesa Verde NP. Nearby is the Four corners NM where you get the opportunity to stand in four states at the same time, the only place this is possible. Utah is also home to some amazing NP's, Arches and Canyonlands near Moab, plus Capital Reef, Bryce canyon [amazing rock spires] and Zion NP. From near here you could visit the North rim of the Grand canyon or head south via Page AZ [Lake Powell, Antelope canyon, Glen canyon etc] to the south rim, or visit both rims !
From GC you can cut across to the coast many ways. A popular route would be via Vegas and Death valley to Yosemite NP [amazing !!] over the Tioga Pass. From Yosemite you could head part way down the coast, say Cambria area and then drive around Big Sur to San Fran although you might prefer LA ?
From SF you could head north over the Golden Gate bridge and visit the Redwoods [Humboltd with the Avenue of the Giants drive as well as the NP] and from Crescent City head inland towards Crater Lake, although access might be limited by this time. From Crater Lake you could head north to Hood river and drive the wonderful Columbia River Gorge into Portland before heading back to Seattle.
There are another 'million and one things' to find along the way, but maybe that will help give you some direction. That would probably come in at around 4500-5000 miles, or 10 days of dedicated driving.
