More experienced cruisers might have more... but before my first cruise we wondered the same thing... What we learned is that alcohol is extra. Probably as pricy as cocktails at a bar. While iced-tea was free, soda was extra. $1.50 per glass I think. Our ship had a Ben-n-Jerry's bar... I think that was subsidized... only about $2 for a nice sized scoop. There was a juice bar that sold slushy fruit drinks for about $4 each.
All food, including room service is included. But there are some extra frilly meals/desserts you can order that will cost extra. There is also the gratuity paid to about half a dozen people who take care of you on the ship. I like that Princess pools the money at a price of $10 per passenger per day (which you can raise or lower at your descression). Then automatically adds it to your room tab. That's easier than having to hand in 6 envelopes of cash on the last day.
There are also some activities on board that may cost a little. We spent $10 per person to attend a photography class. (In a case like this you're really paying for the materials, we got a little hand book.) There might be a pottery class or the like.
There were digital photography services on our ship... you bring them your memory chip every day and they download them to disk.... then at the end of the cruise, you get a CD with your pictures on them.
Oh and speaking of pictures, there are plenty of photo ops where the on board photographers will snap your picture. Your picture (along with hundreds of others) will be hung up on the hallway... if you want it, you can buy any of those pictures. (Along with professional photos of your cruise ship). I forget how much these pics are.
Spa treatments might be equal or a little more than what you'd expect from a standard spa at home.
For what's available, and maybe pricing, you can find your cruise line's web page. I know Princess has a place where I can log in and sign up for spa services ahead of time. I expect the price would be listed ...
Keep in mind...I don't think you'll EVER be using cash money on board. Everything get's charged to your room credit tab.
Hope that helps!!!
Kris