Looking for a list of the top 5 or 10 attractions to see on the way from Portland, Oregon to WAshington, DC. Looking to take a week or so to drive a friends call out their and then fly back. It will probably be sometime in November.
The most direct route is I-84, I-80, I-79, I-76, I-70, and I-270.
Just over 2800 miles; at an estimated 44 hours of driving.
This takes you through Boise, just north of SLC, Cheyenne WY, Omaha NE, Des Moines IA, just south of Chicago, just south of Cleveland OH, and just east of Pittsburg PA.
Do you want to stay on the main route, or are you willing to veer off of it to see more stuff? Staying on the route, here's just a few ideas off the top of my head:
Pendleton Woolen Mills in Pendleton, OR
Oregon Trail Museum in Baker City, OR
Hagerman Fossil Beds, Hagerman, ID
Evil Kneival jump site over the Snake River a tad west of Twin Falls, ID
Golden Spike and various Mormon landmarks (tabernacle, etc) in the Salt Lake City area
Fort Laramie and Laramie Plains Museum in Laramie, WY
Old West Museums and pioneer towns recreations in Cheyenne, WY
Buffallo Bill Ranch Museum in North Platte, NE
Fort Kearney in Kearney, NE
Fort Omaha and Boys Town Museum in Omaha, NE
Frank Lloyd Wright Home & Studio, Jane Addams Hull House Museum, zoo, Wrigley Field, and lots more in Chicago area
Rock n Roll Hall of Fame in Cleveland, OH
Since you're not far, you might veer off-course north of WA DC and take in Gettysburg.
Adding going into Chicago, Cleveland, and Gettysburg will only add about 70 miles to your trip for a total of 2870 miles, and estimated 46 hours of driving.
Hope this gives you some ideas!