Biggest thing is that you should look at these posts then do what fits you.
Some people spend weeks (months?) buying or making little gifts to give to others in groups formed online (fish extenders). I've never done that.
Some people sign up for drink tastings (extra$) or bingo. Never done that.
For little girls there's a boutique experience. Never done that. There are character meetings...some are random around ship but there may be some you schedule. You can also buy photo packages...we did the first few cruises but don't anymore.
My kids did enjoy the various kids and teen clubs. Unless required during Covid, those don't require advance signups. I have done a couple of spa things that were nice (but pricey).
Dinner is scheduled each night in each of the main dining rooms. You choose main dining (usually 5-5:30) or late (usually 8ish) and get your dining rotation when you board. Additionally (and for additional$) you can reserve a lunch or dinner at one of the adult only restaurants. Some people like to dress up for formal, semi-formal or specialty nights. Others not so much. No one will care as long as you look presentable (no swimsuits/holey tanks, etc).
You can sign up for excursions at each port either through
DCL or on your own. Through DCL is more $$ but the boat won't leave you if something happens and you are late getting back. Depends on your risk tolerance.
You can be as busy or as laid back as you like. As a previous poster said....it's vacation.