I have been on a 3, 4 and 7 day cruise. You really don't have to spend a lot of extra money.
1. Plan for tips ahead of time.
2. Stay out of the stores

on board. Or set a strict budget ahead of time (all I bought was a photo album).
3. Take lots of your own pictures and when you see he nice ones at shutters remind yourself that you have all the pictures you will need in your camera ready to be developed at home.
4. Don't go on an excursion. When we went to Nassau the first time we went and walked around a little. The second time we stayed on the ship and we LOVED it.
5.
Castaway Cay. I have to admit, our favorite thing was the bike ride ($6 per person) and we rented a float for each child (2 @ $6 each). But you really don't need a lot of extras.
6. Take your own soda with you for in the room. Don't buy 'drinks' onboard.
7. It is worth a couple dollars tip to order room service one night, so go ahead on splurge on that! The cheese tray is great! Just order ice tea instead of pop because they will charge for the pop.
Honestly, aside from tips, We spent less than $100 total on the 3 day. Now on our 7 day, we did excursions and so I won't even mention what our bill was for that one!
You really can have a great time without spending a lot of money. I was AMAZED at how little we spent on our cruise. It is really up to you. Just plan ahead for tips, and then watch the rest very carefully.
Debra