December 1 is definitely into high season. After two nights in BKK, you'll be bored. Fly down to Phuket, stay at Twin Palms (
http://twinpalms-phuket.com/) at Surin Beach and go hang out with your GF on that beach, Laem Singh Beach (2 minutes away), and Bang Tao Beach (longest stretch of white-sand on the island, I think). If you're after beaches, Phuket and a few minor islands are the only way to go (and, only the West Coast in Phuket, which is where Surin is located). Even Koh Samui's beaches are shit compared to Phuket.
Pattaya is not a relaxing vacation with the GF - that is for guys who leave their GFs at home and take 7 days off for themselves. The Sheraton in Pattaya is shit, as are most hotels down that way (even though there are some nice ones, they are all on terrible beaches). Twin Palms in Phuket is luxury that is affordable and one of top-rated hotels on the island. They have awesome little beach club that has high-grade sun chairs (not the plastic bullshit that you'll see on the rest of the beach), a restaurant with top-notch food, and excellent service.
I lived in Phuket for 2.5 years, most of that on Surin Beach. I know the area like the back of my hand. If you do end up there, check out Liquid Lounge (should still be there), for a couple of drinks, some pool, and gathering spot of island expat personalities. This is a quick walk up the same road as the hotel. Two doors down, a guy named Steven owns a wine shop that hosts events all the time. Hit Supper Club for dinner - best steaks on the island and awesome atmosphere. There is an incredible little breakfast/lunch eatery nearly next to the hotel, owned by a really nice lady who is local but was raised in the US (can't remember the name of that place but it has a wooden patio, you can't miss it). Everything is super-fresh there. There is a killer little bakery on the way over to Bang Tao that makes fresh stuff every day. All of this is in the Surin / Bang Tao area.
Surin area is one of the safest in all of Phuket and, probably, Thailand. It is a bit more upper-class but if you want to impress the GF at an affordable price (and, get some relax time in the process), this is your best bet.
ICQ me if you decide to take that route - I can recommend some of other cool things that you can find in the area (or anywhere else in Phuket, for that matter).