Hey, welcome to Cruisemates!
I've been on three Carnival cruises, all on Carnival Liberty, so I can hopefully answer your questions.
1. Cruise couples: They happen. Just keep in mind that unless you live close by the person or have an airplane, it most likely won't be maintained after the cruise. A lot of people randomly hookup too. It can go either way.
2. Food: The main dining room and buffets have set hours for breakfast, lunch, and dinner. There are snack areas that have things like chicken fingers, burgers, hot dogs, fries, etc... all day. They also have pizza and ice cream available 24/7. As far as drinks go you can get water, lemonade, iced tea, and fruit punch from the buffet after breakfast. During breakfast they have coffee, tea, milk, and different juices. You have to pay for soda or a specialty coffee drink.
3. Excursions: There are a ton of excursions offered through the ship or available privately. If you provide more information on where you're going I'm sure we can help out more
4. Club O2: DO NOT MISS THE FIRST NIGHT. Whatever you do, make an effort to get there. That's when cliques are formed and people fall into their groups. The more people you have, the more groups there are. During the day they have set activities which can be pretty lame at times & at night it turns into a night club scene. It's definitely a ton of fun, and a great way to meet people!
If you have any other questions feel free to ask!