I don't know if you consider tips an unexpected expense, or at least one that you didn't budget, but I think the suggested tips for a 4 day cruise would add up to about $42, I tipped higher then that last time.. it ended up being about an extra $20, but worth it I think... Also keep in mind room service tipping isn't included (we tipped $3-4 depending on what we ordered)
Some unplanned expenses would be:
Spa treatments... they can range anywhere from around ($15 Rainforest Room, to almost $200 a pop)... I did Rainforest last time, you get unlimited one day access to various saunas and showers... We did it during our freeport day--its the cheap way to pamper yourself
Also, any sodas other then the ones you get at dinner will cost you... I think like $2.50 plus a automatic 15% tip..., if you drink a lot of soda, you can get a refill mug for $10, or if you want bring your own to avoid this cost... Alcoholic beverages depend on what you get, I always got the special, and it was usually under $5 (plus the automatic 15%).
Photos... they are always taking your pics, and you have the option to buy them.. they have package deals, but they are pretty pricey, and personally I would have been stretching to try to get the 10 pics for the package... I bought one nice one. Then I just asked the photographer to take a pic with my camera after they took one with the
DCL camera.. I ended up with all the same pics in the end, worked out great!
merchandise/shopping... that's totally up to you...I just bought a few little things..
Finally... there are excursions... they would be an additional cost. I did the snorkeling and that was it, so it was around $25. But they range from $25 to $200.. and then some.
So it's up to you, you could get on board and just pay tips.. or you could spend a fortune.. but there are definitely ways to enjoy yourself, and still not spend a lot. I think including tips I spent about $150 bucks on my last 4 day cruise. And I didn't feel deprived at all.. did snorkeling, rainforest, had a few drinks, bought some stuff, and tipped well too. I can't think of any other expenses really.