Left California for my nephew’s wedding in Milwaukee. I-15 through Nevada, Arizona, and Utah. I-70 across Utah into Colorado. I-76 up to Nebraska. I-80 across Iowa into Illinois, then shorter segments into Wisconsin. Coming back will jog south to Flagstaff, crossing a bit of New Mexico. Final score - ten states in eight days. :)
Crusing through Utah, Colorado, and Nebraska is very nice. Iowa a bit less so - though they have (badly configured) free wireless at rest stops. Illinois and Wisconsin were not so nice (more traffic, not very nice rest stops). California is not nice. Nevada is a bit better. Next trip - stay in the middle (east of California and west of Iowa), and head more north.