If all the "big" things are going on a credit card, then I'd say anywhere between $20-$50 a day, depending on what you will be doing. If you're hanging by the pool & getting snacks, etc., you can put it on the room charge.
If you are in the parks & want a quick snack or ice cream you will want some cash handy. I would think the above amount should be more than enough if all meals, souvenirs, etc. are being put on the card.