We never cruise for less than 14 days. There is lots to do on board and you will not find it a problem. And most princess ships have laundrymats so you don't have to pack extra clothes for a longer cruise.
There is lots of variety of food so even repeats like chicken will be prepared differently. Milk with your meals is not a problem and you can find iced tea and juice as well as tea and coffee.
Many of the computer courses have a fee, but there are often cooking demos, digital photography lectures, language and dancing lessons which do not cost anything. And sometimes there are guest lecturers on board that can be very interesting.
The Caribbean is great for planning your own thing and finding transportation off of the ship. Just remember to give yourself extra time to be back on board the ship.