We are about to go on our 3rd cruise (our first 7 day though). We have done the 3 and 4 day cruise. I spent more on the first cruise because things were 'new' and I had no self control

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On the second cruise we didn't spend that much. I found that after the cruise, I enjoy my own pictures more than the ones from shutters. We really didn't need to buy drinks (I found I could live without Coke, but did bring some for in the room), and my kids were given a budget not to go over (it was their birthday money).
We have never done excursions, but we will be doing them on this cruise. You can plan out your excursions first and add up the cost for them. On board, for the 4 day cruise, we spent less than $100 (not including tips of course). I think it will be somewhat the same this time.
BUT we are on our third cruise, we plan to go every year for our daughters' birthdays (twins), so we don't worry about missing out on anything.....we know we will be back next year!
I did promise my girls they could each get a 'cooler' drink (whatever they are called) on their birthdays this year. But other than that, we really don't spend much on the ship! I am taking some $1's for tipping for room service (can't wait to try the cheese tray and the Chocolate Chip cookies), and some quarters for laundrey (can't pack for 7 days for 4 people, that would be WAY too much to carry!).
So it all depends on what you want to do, and what things you enjoy. There is SO much to enjoy on the ship, you really don't have to spend money....BUT....there are plenty of ways to spend money if you want to. It is all up to you!
Have a great trip! I am anxiously awaiting our cruise next week!!!
DJ