If you live near a store with Disney Gift Cards, I'd buy enough to cover known expenses (ie, Stingray excursion, tips). I suggest this to keep from having holds placed on your card.
Your onboard spending will have a $300 cap. You can pay for your tips and excursions before they are posted to your account and maintain that open limit.
Once the lines at Guest Services get smaller (ie, not on boarding, usually), they can add the cards to your onboard accounts.
[At Sam's Club, a $150 pack of cards costs $143. Target Red Card members save 5% on Disney Cards.]
I've always tipped for room service with cash; a few dollars a visit; however, with a 3-night cruise, you might not need it.
Are you going to be tempted by
DCL photos? A single 8x10 is about $20, last time I cruised. Their professional staged photos are actually a good deal for a family photo, as you aren't having to pay a sitting fee and that $20 photo is a whole lot less expensive than what a studio would charge.
Someone mentioned nursery fees - you might want to check those prices if your little one needs that service.