I guess you can "do" or 'not do" whatever you want. If a list of options were to be created, it would be too large for this forum.
Cruising is a great experience in general but there are some things that are particularly personal. One person's fun may be a drag for someone else. (If you don't swim, you may not want to snorkel etc.)
You can eat at the buffet any time, as long as it is open. You can eat at both dinner seatings if you wish or if you don't want to limit yourself to one entree, you can order two and eat it all or share with your family. You get up from the table and walk away with nothing to sign.
Tipping is appropriate at any time but there is plenty discussed on this forum on that subject.
On the "sail and sign" card, I believe you can block some of your group from using it for anything but access to your stateroom and boarding. Check with Carnival on that if there are some in your charge who you do want to have access to your credit card.