If you're a shopper, I suppose you could hit the outlet malls at Primm/Stateline on the way to Palm Springs from Las Vegas. Otherwise there really isn't anything on the 15 to see.
I would try to do the Grand Canyon from Vegas, much easier than from say Palm Springs.
Palm Springs has an aerial tram ride that might be good for part of a day. Make sure you check the casinos local to the Palm Springs area, sometimes they get some pretty well known entertainers.