Welcome to the addictive world of cruising.
There are a myriad of choices for you and it is hard to choose.
What you have said so far would lead me to recommend a line such as NCL and add $100 to your cruise budget and do some of the alternative dining venues.
Celebrity (In a Solstice class ship), Princess and Holland America would also be good choices. These would be a bit more formal but they have beautiful ships and "overall" decent dining. On many of these cruises they will have lectures on their Alaska, some Mediterranean and longer cruises.
There are many European itineraries that will give you the museums, history and old world charm that you are looking for.
You can also consider doing a Caribbean cruise on one of these lines to see how you like the cruise experience. I would not do anything under seven days. A shorter cruise is a different demographic and experience.
Please come back with more questions that will help us narrow down your needs and expectations and we can make a better recommendation that fits them.