Yes, you can book while on board. If you find an excursion you really don't want to miss, I would advise booking it when you see it. It would also be paid for as opposed to being added to your S&S card. If the excursion is cancelled because of weather, the amount will be credited to your S&S card.
Carnval Mardi Gras a looong, loong time ago
Carnival Victory 2010
Carnival Valor 2011
Carnival Miracle Jan 2012
Carnival Miracle June 2012