Jeans really are not appropriate even on casual evenings. I also have noticed that "casual" has become a pretty snazzy flavor of casual since Princess started using the term "Smart Casual" as the suggested evening attire.
I usually wear a pair of dress slacks and a shirt with a collar when flying, so I can go directly to dinner if necessary when I arrive aboard ship. Ladies, you could do either a casual dress or a skirt or pair of slacks with a top. You probably will find that it's a lot more comfortable than jeans when you're sitting for a few hours on an airplane.
Have a wonderful cruise!