I hope you have a great trip! My fellow United enthusiasts have given you most of the details. Here's what I can add, though:
Yes, your favorite airplane will be quite heavy as Flight 163. There are still a few seats open in First and Economy, but there are more than enough stand-by passengers ready to fill any openings! Your cabin crew so far: 3 Boston-based (including the Purser) and one San Francisco-based. One extra Boston-based Flight Attendant is scheduled to be added by the 24th... most likely he or she will be a junior reserve. Your crew is fairly junior overall.
Inflight, breakfast will be served. You should find the usual "airline choices:" pancakes or french toast and an egg dish. If you're offered our delicious, greasy cheese omelet, by all means take it! You can never go wrong with that one! Why, my record is four in one sitting (I don't recommend trying to beat it)! Along with the hot main course, on your tray you should find a cold muffin or danish and a small dish of cantaloupe and pineapple slices. It's a rather meager offering, so I recommend buying a tasty, fresh bagel before heading to the gate. Before landing, a drink service will be offered with a small package of pretzel mix. One further note: if you are in Economy Plus, I suggest sitting as close to the front as possible (Row 8/9 is the first row in coach). "Regular" Economy begins at row 17. If you're a "regular" guy, try not to sit any farther forward than Row 21, and no farther back than Row 25. All of this strategizing is to simply ensure you get your choice of breakfast dishes, as the more-popular egg dish will almost always disappear at the end of each section of cabin served.
For movies: going to San Francisco, the kid-pleasing holiday fare is "Spy Kids 2." If you're returning to Boston in December, the movie will be "Lilo and Stitch." (United tends to show "family" movies in big holiday travel months.) In January, the Boston-bound flights will show "Sweet Home Alabama." ...While you will be offered a free headset on both flights, I do recommend bringing your good-quality Walkman headphones if you have space in your bag. Our headphones are certainly an improvement over the old plastic tubes we once offered, but they still don't compare with "homemade."
On your all-nighter return flight to Boston, a drink service (and the infamous bag of pretzels!) will be offered soon after take-off. Throughout the flight, the crew should shuffle by in the darkness offering water, and shortly before landing coffee and orange juice should be served... at the mimimum. It's proper and expected to take out the carts with the full complement of drinks, but it's not common practice, unfortunately.
If you have time before your return flight to Boston -- and if you arrive early enough at the airport -- I recommend stopping by the Crab Pot restaurant (next to Gate 72) for a late-night snack. This is the best spot for food in the United terminal, although it will close an hour or two before your flight's departure. If you arrive a little late, the food court near Gates 80/81 should still be open until at least 9 PM. Pizza Hut, Noah's Bagels, and Boudin's sourdough (great --but expensive-- sandwiches) are among the late-night shops still serving. Once on board with a full tummy, you should find a pillow and blanket at your seat for your nighttime flight home (our transcon all-nighters should still offer this for each customer).
Well, that about covers things from this Flight Attendant's perspective. I wish you a good holiday trip. All of us here at UA appreciate your business!