You'll get plentiful answers from the female point of view as what to wear and what is acceptable (and thats great because I am clueless) but what I will tell you is no,,nothing is manditory. The dress code is for the main dining room and there will be formal nights, informal nights and casual nights. If you don't want to dress up you eat at the buffett or even have room service.
My wife and I didn't want to play on formal night so we just brought a coat for me for informal night (she wore a dressy pantsuit) then casual is just that (but jeans would have looked out of place,,dockers for men).
What did feel wierd though , was on formal night walking around the public areas in my casual clothes while the bulk of the passengers were in their formal wear,,but that was in my mind I think,,because no one gave me any dirty looks !
We did do a land tour,,had 4 suitcases and 2 carryones (the max allowed by our cruiseline). You luggage is moved around for you so there is no sweat. As a cruiseline employee told me,,they use the airlinees as their screeners. What ever you can take on a plane,,will fit on their busses.
You know whats important,,,have fun and do what you feel like doing!!!!