You'll want to spend a day or three around Rapid City, SD. The Black Hills are wonderful for motorcycle riding. Consider going there during the Sturgis Rally if that's your thing. Just be prepared for seriously high prices for everything. Hotel rooms, food, even campgrounds get expensive during the rally. If you're an introvert like me and a million motorcycles packed in as tightly as possible with cops around every corner sounds like a nightmare, go just before the rally. June is a good month to be there. Not too hot. Not too cold. Not too crowded.
Check out this route for a great ride. Keystone is a cute little town. 16A is good and twisty with great sight seeing opportunities. It's hard to beat The Needles Highway too. And of course, Mt. Rushmore is right around the corner for some iconic Americana. (In fact, I'm the Black Hills area right now. I can get some pictures of the area when the rain stops if you want.)
If you're going from Rapid City to Seatle, you should make a detour down the Beartooth Highway. It's south of Red Lodge Montana, which is south of Billings. It's another road that's worth a ride. Great views as far as the eye can see. The Beartooth Highway will take you from Red Lodge all the way into Yellowstone National Park, another place you should spend a couple of days. A little bit of Google searching will show you how amazing Yellowstone is. You could easily pick a million worse places to spend a few days.
Then when you head down from Seatle to Southern California, take Highways 101 and 1. It's not as fast as I-5, but they run along the Pacific coast. The Oregon and California coasts are wonderful for sight seeing. Oh, and 101 will take you over the Golden Gate Bridge. I-5 won't even get you close enough to see it. While you're in SF, consider a tour of Alcatraz Island. I've never done it, but I'd love to.
I grew up on the Gulf coast, so it all feels stale to me. Nothing really worth seeing there for me, so take other people's suggestions over mine for that area. Instead, I'd head over to Utah. Rent a Jeep in Moab and explore some of the off road trails in Arches and Canyonlands National Parks. Then head south a little over to Zion National Park. You can hike up the river that carved the Zion Canyon. It's a unique experience for sure! From there, head down to Carlsbad New Mexico. Carlsbad Caverns (another National Park) is another of my favorite places to visit. It's very comfortable in the cave at about 13 C all year long. Try to get a tour of the Lower Cave for some less traveled caving fun. Or if you are really adventurous, try the Hall of the White Giant. You crawl through tight passages, up some slick rock, and will get dirty doing it. Just don't do it if you're a bigger man than me. I was about 5'9" and 225 lbs. when I did it, and it got a little tight for comfort.
From there, you hit the Gulf Coast. Just get on the bikes and make good time down to the Florida Keys. Consider taking the ferry over to Dry Tortugas National Park. It's an old military fort that never really got finished, but it's surrounded by amazing turquoise water and coral beaches. You can even tent camp there overnight. What's more epic than camping on a coral beach of a tropical island in an area with no light pollution? If you're a tent camper, that's gotta be paradise. Make sure to reserve early though. Tent space is limited. Then when you get back to Key West, take some pics of your bikes by the Southernmost Point Marker. It's a great photo op!
Then head north to Deal's Gap where North Carolina and Tennessee meet, and ride the infamous Tail of the Dragon! Get your picture by the big dragon statue and tree of shame. From there, head up the Blue Ridge Parkway and Skyline Drive. That will get you all the way from Cherokee North Carolina to Front Royal Virginia. Some of the best views you can imagine can be found on those two roads. Nice riding roads too.
After that, maybe head over to Virginia Beach for some nice R&R. Or up to Philadelphia for a real Philly Cheesesteak. Or maybe you're looking for the night life so a little farther to New York City. Just keep in mind that the contiguous US is freakishly big. You're looking at 10,000 km at least for a trip around the US.