My wife and I just returned from the same cruise itinerary on Navigator. Same age group. There is more than enough entertainment on board to keep you busy for 5 nights. You don't want to miss the ice show.
We opted for dinner at both Portofinos and Chops. No regrets. Gratuities were included, drinks were not. All other meals were at the Windjammer so no need to tip the wait staff upon departure.
If you like to drink alcohol you can run up a tab pretty quick. I love beer. $4.89 per Budweiser (includes tip). I left a $20 cash tip for our bartenders at both the Sky bar and Boleros.
We also left a $40 cash tip for our stateroom attendant. Total cash tip bill was $80.
$30 for a memory card for the camera. $35 for a pair of sandals. $30 for a carton of cigarettes. $?? gifts for family and friends. $40 on the slot machines. You'll want to purchase pictures which are taken when exiting the ship at ports of call. $20 for a photo album to hold the pictures.
In the end the total expense registered to my SeaPass was $800.
There is very little time spent at ports of call. So, little time to spend money. We spent $45 on liquor in Gerogetown, and $70 on trinkets in Cozumel. If you want to visit the beaches you'll need cab fair. $3 per person in Gerorgetown. $7 per person in Cozumel. Beware of shopkeepers in Cozumel. They are exremely annoying.
What's a trip to Mexico without eating Mexican food? We had lunch at a place called El Abuelo Gerardo. $33 including 4 drinks and tip. Great food, great price.
I can go on for hours, but I hope this gives you an idea of the additional expenses.