Based on my experience, I'd say wait until day 3 to book your spa massage..there will be great deals after shore excursion 1.
The cabin service was completely invisible. I only met my steward once on the first day. Not once after that did I even see him in the corridor.
LOVE the carnival towel animals in the cabin every night.
Carnival did an AWESOME job working with the low-carb group. I would suggest that as the cruise gets bigger, it would be more important to provide more low-carb options in the menu.
I would have probably spent $50 at the park, but the vendors were so aggressive, I didn't spend a penny.