Let's see...
Formal night. Formal night is on all cruises that I believe are 7 nights or longer? Maybe 6 nights? I think it's 7. Anywhoo, it's typically the night of your first at-sea day. My theory is they want you to still fit into your fancy clothes you brought from home. LOL! No, you do not pick which restaurant you want for formal night per se....which leads me to....
Dining rotation. This is something I believe Disney introduced to the cruise industry. On each of the ships there are 3 different themed main dining rooms (MDRs). When you receive your room key you will see a series of letters on the key. Here's my room key from our recent 8-night from Galveston:
See it under the words SILVER CASTAWAY? It says: "Dinner: 8:15 PM-LAAPLAPL". Translated for you, this means my dinner seating time in the main dining rooms is 8:15 pm. The letters are the restaurants in order for each night of the cruise. We had Lumiere's, Animator's Palate, Animator's Palate, Parrot Cay, Lumiere's, Animator's Palate, Parrot Cay, then Lumiere's. This is so you always know what your assigned restaurant is. One thing you'll always have during the cruise is your room key, right? So each night you rotate to a different main dining room at your assigned time. Your servers will rotate with you. So no matter which of the main dining rooms you rotate to, your wait staff will be the same for the entire cruise. I love this because they get to know you. They start to understand your preferences. A few nights in it's not uncommon to arrive to your assigned table (they're all numbered and you'll always sit at the same table number in all main dining room dinners) and have your usual beverage selections already on the table waiting for you. Or, when it's dessert time they will appear with the dessert you ordered plus a plate with another dessert you commented that you really liked. They get to know you which is awesome! And that leads me too....
The gratuities. The gratuities will be paid to your assigned wait staff. No need to worry about the right person getting the tips. They've been with you the whole cruise. Their name is on the little tickets that you will be given that you put in each corresponding envelope. We generally tuck some cash in with the prepaid tickets as an additional tip if the service we received warranted it.
There are adult-only dining venues that you can book if you so choose. These are the specialty upcharge restaurants onboard. On the classic ships that would be Palo. On the new ships that's Palo & Remy. These you book the same way you do excursions or other extras. You can choose from available days & times when it's time to do all the fun booking of extras when you get closer to your time of sailing. For new cruisers that will begin 75 days prior to sailing after you are paid in full. Which conveniently leads to....
Excursions! There's a wide range of offerings in terms of activity levels, types of excursions, and costs. It's up to you to read thru them and weigh what's worth it to do. Some, yes, are quite pricy. It's like going to the parks in that you can buy add-ons (such as tours, premium dining experiences, etc.) depending on what kind of money you are willing to spend. If you heard about a cool VIP-type experience at WDW you prob'ly wouldn't just book it and throw money at it blindly. You'd research as much as possible first, right? So when picking thru the lists of excursion offerings go out on the interwebs and Google those things. Go to YouTube and search for related videos to see different perspectives. Come here and ask if anyone has done that excursion before. Find other cruise or destination specific websites that might have more people to ask about it or who have already provided reviews. Trip Advisor and Cruise Critic are great resources. The cool part about booking far in advance is that you have more time to study the offerings and decide what to do in each port call. Even if you don't schedule a thing you have the opportunity to step off the ship and see something you might not have seen before. For the bargain price of free you've got a whole new world to explore. That's awesome!!!
The FAQ thread here is great. Lots of great info that you can readily pick thru to answer the questions you have. And, of course, you can always ask and plenty of folks are happy to help with responses.