Just don't go to Iowa... nothing there but cornstalks.

Head north to some beautiful riding in Minnesota/Wisconsin. Take the trip up (or down) the Mississippi River. Beautiful scenery, great roads, fun pubs on the way (if you're so inclined).

I was fortunate enough to do a little riding in the Chianti region a couple years ago during a trip to Firenza. You got some great roads over there, too!

 

Alberto,

Everyone has listed a lot of great rides and places to go. Let me suggest some things to do while you are here that will give you a taste of
America.
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If your arrival port is to be Charleston, be sure to take a carriage ride tour of the city.
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If you head north/northwest to the Blue Ridge Parkway you could visit The Great Smoky Mountains National Park. Do some of the hiking trails. On your way there, take route 11 west through South Carolina and stop at a peach orchard/farm. If they are in season you have never tasted a better peach.
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Pick up the Parkway at the Smokeys. Order a map of the parkway through it's web site. You will need one in order to know where to get on and off for food/fuel/lodging/etc. since the Parkway allow no advertising signs.
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While on the Parkway, stop off in Roanoke Virginia and stay a night at the Roanoke Hotel. Grand old place with a really good restaurant. The city has a small railroad museum that is interesting and doesn't take long to see.
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Further north on the Parkway stop at the Peaks of Otter and hike to the top. You can see forever.
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In Pennsylvania stop at Gettysburg National Military Park and learn a little about the Civil War (or the War of Northern Agression as it's called in the south).
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Close by in York is the Harley Manufacturing facility where you can tour the plant.
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I'll let others tell you about things to do in the northeast but:
If you find yourself along the southern shore of lake Erie in the state of Ohio, be sure to take in a Cleveland Indians baseball game. I'm pretty sure you can get tickets at the gate.
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Also in Ohio, if you are at all in to roller coasters, you must stop at the Cedar Point Amusement Park. They are recognized as the best in the world and have something like a dozen coasters. Plan to stay an entire day.
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Also along the north coast of Ohio, visit Kelley's Island. Take the Ferry over. Leave your bike at the dock as you can walk anywhere you want to go on the island.
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Then head north through Michigan and be sure to cross the Macinack Bridge. As soon as you get to the north side, stop and take the ferry across to Mackinac Island. Spend a day there. No motorized vehicles allowed. Only horses and bicycles.
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While on Michigans Upper Penninsula, be sure to go to the northwest side and take a boat ride to view the Pictured Rocks National Lakeshore.
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I'll stop there. Maybe someone else can jump in. Have a safe trip.

 

He is taking 3 months, so figuring 6 weeks out, 6 weeks back should afford lots of time to see everything. Most everything that is....lol.

 

I'll add a couple cents worth on the U.P. and northern Wisconsin. The ride up the Queens Highway 17 north of the border from Saulte Ste Marie (the Soo) up to Wawa, Ontario is a beautiful ride on the east end of Lake Superior. I would do an up and down back to the Soo. the ride up to Whitefish point with a stop at the Iriquois lighthouse is nice. Taquamenon Falls near the point is pretty impressive before heading to Munising and the Pictured Rocks. I believe the new county road is open between grand Marais, MI and if it is I would go and stop at the Lake Superior Brewing company and try an Agate Ale. Good food, really interesting interior at the bar with table tops covered with sliced agates. A boat ride at Munising to view teh pictured rocks is best in the afternoon when the sun is shining on the rocks. there's also a glass bottom boat to view shipwrecks at Munising called Shipwrecktours.com. From Munising I would make a run west through Marquette to Lanse and to the Keeweenaw peninsula to Copper Harbor going up on US 41 from Houghton and back along the lake on Michigan hwy 26. From there I would ride over to the Porcupine Mountains and see the Lake in the Clouds. After leaving the U.P. of Michigan I would follow US 2 through Ashland and up WI hwy 13 to Bayfield and visit the Apostle Islands. From Bayfield continue on 13 to Duluth-Superior and visit the Canal Park area. From Duluth a run up HWY 61 to the Split Rock Light is very scenic. From Duluth you could head west through Minnesota to the Dakotas to see the large Reservoirs on the Missouri River, the Badlands, Sturgis and Mount Rushmore . There's where someone else can take it.

 

Hey, great idea, that`s the road maps I normally use: http://vectormap.info they also provide some freebie staff as well

 

Everyone has suggested great places to see. Top of my list would be the mountains Wyoming, Montana, and Colorado. Those areas you will find Parks and areas such as Yellowstone,Glacier,Grand Tetons. Bearthooth Pass and Chief Joseph Hwy. are great places to see in that area. One place I would recommend is the Black Hills where you will see Mount Rushmore, Crazy Horse and Needles Hwy. You may want to plan your trip so you can be in Sturgis the first week in August. One last thing, The United States have lots of great things to see. But it is a very big place with Hundreds of miles of nothing between the good spots.

 

Yellowstone, Glacier park in Montana, Death Valley, Grand Canyon, California and Oregon Coast, Banff and Jasper. Plot out a loop around those and fill in the areas with good connections.

The best way to see the Oregon California coast to South. The view and the parking is to your right.

If you read my thread from this summer, you will see that is pretty much what I did.

Enjoy

 

1) Banff and Jasper and Columbia Icefields Parkway between Banff and Jasper
2) Vancouver to Whister to Cache Creek is a great ride
3) Glacier National Park, road to sun is beautiful
4) Yellowstone National Park Area
5) Grand, Bryce and Zion Canyons in Arizona and Utah

 

I'll pick up this thread because a 90-day ride across USA is definitely on my bucket list.
Add the fact that I'm close to retirement and you can ju-u-ust see the corners of my mouth curling into a smile...

Unfortunately rentals are so expensive in the US it is actually cheaper to crate&ship than it is to rent for long hauls; a week or 15 days I can afford, 90 days and I am close to buying the bike outright!

My idea is to ship to Boston or NYC - I'd rather avoid Baltimore.
From there, New England, Virginia (where I lived as a kid), Blue Ridge and Appalachians and after a brief dash to Cape Canaveral, it's New Orleans.
Then due north up along the Missississippippi (got lost there...) to Chicago, down Rt. 66 to Santa Monica (see Grand Canyon, Death Valley and Las Vegas along the way), see Sequoia&Kings Canyon, Yosemite&Sa.Francisco, across Tioga Pass into Nevada, Salt Lake City and up to Glacier&Yellowstone.
Due east across the Black Hills back into Harleylandia, stay awake across Iowa Illinois Indiana and Ohio, wade across corn fields into Pennsylvania, back to port, crate&ship back home.

I'm writing this out of my head and I well know that even the most skewered fig.8 route will never ever get me to see all the sights - still, time is a limit.
Of course I could head due west after seeing St.Louis, come to think about it there's no reason for me to ride all the way from New Orleans to Chicago... just to head south once again...

The generic European tourist can stay 90 days with his "Visa-Waiver Program" stamp on his passport.
Unless, of course, one of you gentlemen knows someone who knows someone at Immigration, because...
...if at anyone point in my trip I decide to cross the border into Canada, ride there for a few days and re-enter the US from any other border crossing... would my 90 days restart from there ?
It would be one nice and legal way to prolong my stay without actually having to fly out and back...

 

Here you can start with something like this, it hits pretty much the best places in the USA