Here, I'll answer your questions. I was on the Destiny 10-5-09 for the Eastern Caribbean cruise and it was awesome :).
What is usually worn at the informal dinners? Someone told me that sun dresses are fine.
>> Yeah, that would be fine. I usually wore capri pants and a nice shirt. My boyfriend wore jeans and a polo. As long as you're not wearing a swimsuit or swim cover ups you're okay though. Some people wore shorts even.
What does it mean to have a carry on bag? We're only taking on suit case each and we're packing light.
>> I would bring a carry on bag to use when you go off the ship, because you'll want to take your carnival beach towel, sail and sign card, sunscreen, water bottle, things like that. When you get on the ship they take your suitcase and they deliver it to your room that night, so you'd want to have the carry on to hold things such as medication, sunscreen, toothbrush/toothpaste, makeup, or whatever you might need. We got on the ship at like 12 and didn't have all of our luggage until around 8.
Are there towels at the excursions?
>> Some do (Atlantis does, I know), but most of them they wanted us to bring our blue Carnival beach towels that they leave in your room. I guess they'll tell you if the excursion has beach towels, but you don't really need to bring one because they're provided (however, if you want to bring one it's not a bad idea because if you lose the carnival beach towel it's like $22 so yeah, if you don't want to take that chance then bring one).