There's plenty to see on the way, if you're willing to divert off Interstate Highway 15 for part of the way.
You are going right through the greater Los Angeles Area, and you could easily see the Getty Museum, Gene Autry Museum, La Brea Tar pits, Mission San Juan Capistrano, or any of the LA tourist spots,
Disneyland/Cal Adventure (of course), Universal, Knotts Berry Farm, Magic Mountain.
There's Big Bear and Lake Arrowhead.
There's the San Bernardino County Museum:
http://www.co.san-bernardino.ca.us/museum/exhibits/exhibits.htm
There's Palm Springs and the Aerial Tram
There's Temecula Wine Country.
There's any beach from LA, OC, or San Diego.
There's Griffith Park.
There's the Wild Animal Park in Escondido (nope - it's not in San Diego City).
There's Mount Palomar Observatory.
There's Ontario Mills Mall, Victoria Gardens, Bass Pro Shops.
Wanna Parachute Jump? You can do that on the way.
Wanna Balloon Ride? You can do that on the way.
Boating, Fishing, Hunting, Skiing (water & snow), Sporting (baseball, basketball, golf, football (American & European), hockey, skating, gymnastics). Big Bear has an olympic training facility, and is a training home to gold medalists. The Little League World Series is played in North San Bernardino.
That's just off the top of my head.