I was part of a group of over 120 people with a modified diet (low carb). All meals were doable, but I would have like it if Carnival would have planned more in advance. I.E. most low-carbers would LOVE berries for dessert. We got it for the first night, but after that we were informed that it was not an option. I LOVE the fact that on a few nights they made a low-carb cheesecake, but the fact is that NOTHING is a replacement for simple berries.
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.