Why not discuss taking a trip that's been on the back burner.  Either an R&R or a tour depending on her tastes.
Broach the negativity subject when you get home.  Ask how she's adjusting to her retirement and what to substitute for her career challenge.  Never use the word 'negativity'.  Never use the word 'happy'.  Saying she doesn't 'seem like herself' with a few gentle examples will communicate your point.  Don't say, imply or suggest that you want to help her 'fix' her problem.  Women hate men who want to lunge in and fix things.  Offer your services by saying, "Is there something I am doing that I need to change?"  Or "something you want to talk about?"   Has she taken over all the drudgery now that she is retired?  Are you going out at least once a week.   Do you want her company in the home?  Do you have movie night? Hors d'oeuvres?  Getaway weekends?