Simple. Go to a decent travel agent, tell them what you want & they will arrange it. For instance fly to Vegas, spend a few days there, then 30 minute flight to L.A. for a Mexican Riviera cruise. Alternatively fly to Miami with even greater choice of cruises.
Cunard do tailor made Las Vegas add-ons, staying at The Luxor on the Strip. It's a high end hotel, well situated and the whole package is reasonably priced.
WE are going on an Alaskan cruise with Royal Caribbean September this year which repositions down to L.A. We have delayed our flights back and are spending a few days in L.A. and Las Vegas. I'm sure they will be doing the same next year.