In Niagara Falls, go to the Canadian side...much better view. Make sure you spend one evening, so you can see the falls under the lights. Really cool butterfly zoo, also on the Canadian side.
The US is big! I've driven straight across, took 7 days, w/ no sidetrips. Yes, it was a bit slow, since The Mrs. couldn't drive the truck, and we were towing a car.
San Francisco is well worth the trip. If you can, do Yellowstone (not too far from SLC). Betw SLC and Vegas, try to get to Lake Tahoe--spectacular. While you're there, you might as well do Yosemite as well, before going through the Sierra's down to Las Vegas.
In NYC...Ms. Liberty is worth a trip, the Museum of Modern Art, goto the ticket place in Times Square and get 1/2 price tickets to a show that day. The seaport at South Street, the Carrier on the lower west side, Grand Central Station, the Empire State Building, the Chrysler building, the Guggenheim museum, F.A.O. Schwartz (toy store) next to Central Park, etc... Often, you do well to take a bus tour around the city, since it gives your legs a rest.
What are your interests?