I live in Vegas and have APs to Disneyland so we make that trek often. It usually takes us about 4 and a half hours, and that includes one brief stop to eat.
I just wanted to say that we've seen several horrific accidents along the way, and my DH who works at a local hospital, has seen several people helicoptered in from along the 15 freeway. Not the way you want to arrive in Vegas.
So please be careful out there. We want you to arrive safe and sound.
As far as great things not to miss, well, I wouldn't call them great, but there are things to see. In Barstow there is an In-n-Out burger as was previously mentioned. Good place to eat. There are also outlet stores there.
In Yermo, just past Barstow, there is Peggy Sue's Diner. Never been in there but I've heard the food is good. http://www.allaboutthefun.com/ALLABOUTTHEFUN/Peggy_Sue's_50.html
Also along the way is Calico Ghost Town. Same exit as Peggy Sue's I think. It's something kind of interesting to see, if you have the time. We stopped once with the kids and they liked it alright.
http://www.calicotown.com/
In Baker, also known as Gateway to Death Valley, you'll find the Mad Greek, the world's largest thermometor and some "Alien Jerky".
http://www.roadtripamerica.com/eats/madgreek.htm Their food is pretty good. They are known for their fresh strawberry shakes.
We've had their chicken Gyro:
Here is the Alien Jerky store:
Other then that, it's pretty much a boring drive. You'll see lots of this:
Hope that helps.