That's quite an itinerary, if I may mildly second-guess it I would say your friends are going to be sore and exhausted after that trip. New Year's Eve in Las Vegas is very busy, if they don't have a hotel reserved yet they should do so soon or their choices are likely to be limited (and expensive). The drive to Yosemite and the Grand Canyon in late December is also very dependent on weather; frankly the Grand Canyon is covered by fog during many of the winter months and so while it can be a relaxing trip and a stay at El Tovar in the winter can be fun (if you can get in over the holidays), blowing in for part of a day to see the tops of clouds can be . . . less than rewarding.

The problem I see in the Yosemite-Vegas-Grand Canyon part of the trip is they're going to have to make some expensive hotel reservations that they might or might not be able to get to. So plan accordingly.

Also there is very little going on in Hollywood January 2 or 3, most television productions (even without the WGA strike which won't be settled by then) will not be up and running due to the holiday hiatus/strike. The Universal Studios tour should be operating but there will be relatively little to see for the same reasons.

Sorry for offering depressing thoughts on what sounds like a fun trip for them. I might suggest saving the Grand Canyon for a spring/summer trip and eliminating one or the other of Yosemite or Vegas, just for the sheer sake of having some time to enjoy what they do see. You can easily spend 2-3 days in Yosemite alone. If on the other hand they're young, energetic and can make alternative arrangements if the weather doesn't cooperate, they can certainly say they saw a lot in their 10 days here.