Drink/Alcohol card: It is a card you buy that gives you a certain number of a type of drink. There are softdrink cards, alcohol cards (which I think is mainly beer, but I may be wrong), etc. I think you can buy them at the bar, but you should be able to get them at the pursers desk (customer services). Price, availability, and number of drinks per card vary by the cruise line and seems to be in a state of flux.
Room service on all ships I've been on is 24/7. The menu varies by ship, but it is very limited in comparison to what you can find at the buffets and main dining. Some lines only have sandwiches, continental breakfast, pizza, etc. whereas other lines have a fuller fare, for example, on Celebrity you can order any dessert left over from dinner (even though it isn't on the menu).
Drink and beer prices are high in my opinion, but then I don't drink alcohol. I think they are similar to what you would find in a typical bar in major US cities ($3-4 beers, 5-7 drinks). 15% gratuity is added automatically, even for soft drinks. Some lines do have buckets of beers (individual beers in a bucket of ice). I think you get 4 for about $12-15, but again it varies by cruise line.
Carnival has some of the largest rooms in the cruise industry. Not sure of how their balconies stand up, but I suspect they may be a tad larger due to the larger width of the cabin. However, RCCL has some of the best cabin design in the industry which mediates the smaller room size. I've been on Adventure, and the rooms are great. Lots of storage, and you get a curved shower stall door instead of a curtain (more room, and it doesn fly in and stick to you when someone opens the door.
RCCL is my favorite line (I've cruised Carnival, Princess, Costa, Celebrity, RCCL), and of RCCLs ships, the Voyager-class is my favorite. Of the Voyager-class I've sailed on, Adventure was my favorite. Definitely check out the men's restroom on the 11th deck I believe. It has a full glass wall facing the ocean and a waterfall urinal. The removed the waterfall on ships after Adventure.