It's a bit different on a
DCL cruise. Instead of one main dining room, there are three separately-themed reastaurants. For dinner, you do sit with the same people at the same "table," that is, the same table number, but you rotate through the restaurants each night of the cruise. Your tablemates and wait staff rotate with you.
For example, on the Magic, you might eat at Lumière's the first night, Animator's Palate the second night, and then Parrot Cay the next night, starting the rotation again on night #4. You'll have the same tablemates for dinner all week.
For breakfast and lunch, they do not use assinged seating. Usually one of the sitdown restaurants is open for each meal, and the wait staff seats you as you arrive, filling up tables as they go. The original poster's question referred to breakfast and lunch. Since there are other options for these two meals (Pluto's, the buffet, room service, Pinnochio's pizza), there aren't as many people in the dining rooms at once, so they try not to use all of the tables if they don't have to.
Regarding excursions, you'll get some info with your cruise documents, but you can call and schedule them in advance (up to 60 days prior to your cruise). The DCL Website has info on the excursions offered
here.