As a surprise, pre-pay some type of "welcome aboard" package Have it delivered to their room 30 minutes after the ship leaves port

We sent a champagne package to my brother and his wife on their anniversary cruise Of course, we didn't tell them it was coming until the basket just showed up They couldn't stop talking about it

Or you can pre pay a bottle of wine for a dinner, or one of the shore excursions

The cruise line will help you pick something appropriate