Bakersfield is "it" if you want something completely on the path. It's the jumping off point between NorCal and LV, is a real city so has all the necessities of civilization (chain hotels, McDonalds, etc.). But if you want to take a day and smell the roses on the way:
Disneyland. Santa Monica. These two aren't really on the way, but if you want to do/see something as part of the layover...
Santa Barbara is a nice layover, but like Disneyland is best enjoyed with a day to itself, and it's off the path.
If you just desperately want to get closer to Vegas, there's Baker. But really if Bakersfield isn't exactly a destination, neither is Baker - it's a small town of last resort for weary travellers, like Bakersfield, only out in the desert. Avoid Arne's Grand Hawaiian Hotel.
I'd go for Santa Monica - walk the promenade, put your feet in the beach, there's a variety of places to stay. It's out of your way, true, but makes for a nice break. Brave the LA traffic and enjoy a restful evening. And you won't feel like you're missing anything if you only stay the night.