Unless you are in the posh cabins you cannot book a table in the restaurant until on board. You need to be go to the restaurant reception on deck 7 to book. Do this pretty early or you might have to go on the reserve list and wait to be called once tables are free. About 43 euros for 4 courses, drinks extra. There is an alternative cafeteria.
No idea what meds you will need. I don't get sea sick. Rough crossings are great as there is no queue in the bar.
If either of you are interested in looking out for whale and dolphin, there will be a briefing on the main bar on deck 8 with someone from a marine conservation charity.
Wifi: you get a free hour per device so having a tablet as well as a phone is handy. Otherwise put devices in airplane mode so you do not rack up marine cellphone charges.
There's a small indoor pool on deck 9
If you want a drink without enduring the cabaret, there is a quiet bar on deck 7 near the restaurant.
Don't be in a hurry to go to deck 2 on arrival. You have to wait for the upper deck to clear before they open the doors, some twat will start their engine too early and there is a bit of a scrum to get off.
Don't plan to do many miles on the first afternoon. Lumpy weather can cause delays so stay within a couple of hours of Santander.