Well, for something interesting, you might want to look into taking US 6. At one time, it was the longest route in the nation, and used to run from Long Beach, CA to Cape Cod, MA. It actually ends in Provincetown at the tip of the cape. The stretch from Long Beach to Bishop, CA has since been removed from the highway (most of it is now only identified as Hwy. 395). This could possibly be a long trip, but I think from a nostalgic perspective, it would be worth it. I hope to take this trip one of these days.
By the way, in many places, US 6 is the same road as some Interstate highways. For example, through parts of Utah and Colorado, it is the same road as I-70. There are places where it diverges from the Interstate, like around the Eisenhower Tunnel, for instance.
Here's a link with some interesting info:
Finding US 6