Claudia,
I wrote the Azores guide, available at the top of this Portugal forum, and the actual post was written by the webmaster The freemium available in it will have lots of Azores answers.
The weather in December is never consistent year to year. At times our relatives can be swimming in the ocean and sunbathing. Other times, the weather can be terrible. Pack with the intent to wear layers, and bring an umbrella and good hiking boots. Keep an eye on the 10 day weather forecasts before you arrive.
Some restaurants I went to this past August are Quinta dos Acores, Xitaka, Restaurante Alcides, Restaurante Fontenario, and I would highly recommend them. We went for dinner one night at a restaurant in Parque Atlantique (Donatello) and it was fantastic. “A Tasca” and Mariserra are family favorites. There are others around the island that I can also recommend…Alabote, Botequim Acoriano, Miroma, Tronqueira, O Silva, Associacao Agricola de Sao Miguel, Queijaria Furnense, and Restaurante Casa De Pasto O Cardoso.
Mercearia Sao Pedro Wine & Tapas is good for a casual, low key evening and Trianon has an excellent lunch buffet. Cafe Royal is my go to spot for bifana sandwiches. There are cachorro carts (hot dogs with all the fixings) at night along the harbor, especially across from Cais da Sardinha. All the restaurants in the Solmar building are good and inexpensive, along with O Rei dos Pregos next door.
“O Cholcolatinho” has amazing ice cream and desserts.
My driving day trips, self guided walking tours, and much more information are in the paid version.
Send me a private message with any other Azores related questions or concerns.
Carl