What a nice husband
If you sign in to the cruiseline website, they should have some info on the spa. If you can sign up for massage at the website, you will see that option. Otherwise, go directly to the spa when you get on the ship and book then - reservations fill quickly. When you have a full day or two at sea, that is a good time for pampering and relaxing. You can generally get a 30-min, 1-hr, or 1-1/2 hr massage. I always spring for the most I can get - 30 minutes just barely gets you relaxing. Your wife goes to the spa where they usually have dressing rooms, lockers, showers, etc. She will change, put her things in a locker, don a robe, and go for her massage.
You can select a full-body massage (my fave), or partial (feet, shoulders). There are hot rock treatments, wraps, facials, etc. If you have no idea what your wife would like, then maybe go for a 1-hr massage, then another appointment for a facial. Just a suggestion. The 1 hr massage will run about $100 or so.
I've only said Norwegian and Princess, so maybe others would have specifics on Carnival for you.