I stayed at the ibis Gdansk. It's an ibis, but in good condition, friendly service. The location is close enough to walk to the old town and see the sights. Not sure how much time you'll spend on the beach in Dec, so I'd recommend Gdansk over Gdynia/Sopot at that time.
I also spent time in Wroclaw and Poznan, both cities worth visiting if you are planning to tour the country - a lot fewer tourists, but plenty to see & do...