I will be getting on the ship when you get off, on January 20, and will be on it for four weeks. Actually, I've been on two cruises on the Mercury in the past year, both ones for 2 weeks each, so, as you can tell, I love the ship!
I think the amount of and participation at evening activities depends a bit on the intinerary, but we always found that after a full day of sightseeing, dinner, the show and maybe listening to a band for an hour or so in one of the lounges, we were ready for bed. They do have several music choices in the evening that should appeal to you, and, for your daughters, they have a disco in the Navigator Club (although I'm not sure how well attended it is...seemed to me most of the people were entertainers from the ship). I don't know if Opus (the party band) will still be on the ship, but I loved them, and could have listened to them till 1 am every night, if sleep wasn't a factor.
The ship is beautiful...not glitzy, at all, but very classy. The food is first rate (although, admittedly, I'm not a goumet at all...any meal I don't have to shop for, prepare, or clean up from is good eats to me). The buffet is terrific, and one of the most popular items is a made-to-order pasta everyday at lunch. They have the BEST homemade ice cream at sea (IMO), and they usually have different flavors offered in the Palm Springs grill vs. the cafe (you'll see what I mean when you're on board).
The pool area is very nice, although I'm anticipating that it may be way too cool to use it several days between Valaparaiso and Buenos Aires. The thallasotherapy pool is also nice, but, be careful...it eat bathing suits (wear your old one in there, if you use it). The steam room is wonderful...lots of Italian tile. Unless you are in a suite, there is an extra charge to use the spa area, but, we're planning to pay for it, especially given the colder climate. There is no charge for the exercise facilities.
We've always had an inside cabin, up high on the ship. They are very comfortable, and most important to us, have large bathrooms, roomy showers, and lots of storage.
I've heard (from others on the internet) that there will be alternatives at each of the ports to the Celebrity shore excursions, and we aren't pre-booking anything at this point. I think shore excursions are Celebrity's weakest point, but they are on par with other cruise lines we've been on.
Service was wonderful, both in the dining room and the cabins. The staff always seemed like they didn't mind what they were doing, and loved to be of assistance. I've been on 6 cruises, and the two on the Mercury were my favorites.
Hope this helps...leave us some food on the ship,ok?
Have a great cruise!