What are your favorite onboard activities during sea days

I have my first cruise coming up soon and I am really looking forward to the sea days but I want to make sure I do not waste them being bored.

What are your favorite onboard activities during sea days? Do you enjoy trivia sessions, pool deck time, live shows, spa treatments, fitness classes or something completely different?

Which ones turned out to be surprisingly fun or which would you recommend skipping?

Any tips for first timers to get the most out of sea days would be great.