I have lived in Carslbad and although there is not a whole lot going on in Carlsbad itself, it is relatively cloe (30-40 min drive) to downtown and there are a lot of nice areas between Carlsbad and Downtown SD.
In Carlsbad itself, the La Costa Club/Resort is nice and it has a pretty decent restaurant/bar that I believe is called the blue fire grill. Pricey, but good food and a nice bar with an outdoor, heated patio overlooking the golf course.
About 10 mins south of Carlsbad is Encinitas, a nice beach town with some nice restaurants and shops. For a hole in the wall, but delicious Mexican place, try Juanitas on highway 1. Also in Encinitas is Kaito Sushi, a very authenticN solid Japanese restaurant a few minutes inland and a great Tapas restaurant called Q'ero. Moonlight beach is a nice beach, but I don't imagine you'll be swimming or surfing this time of year.
Del Mar, about 20 mins south of Carlsbad has a nice little shopping area known as the Del Mar Plaza. It is a nice place to walk around and there are a few bars in there as well. A good restaurant in this area with a nice view is Pacifico.
As mentioned above, La Jolla is a nice area and I recommend you spend a day there walking along the cove and hitting some of the shops and restaurants on Prospect st. This is really a beautiful area about 30 minutes south of Carlsbad. Although I have never eaten there (only drinks) the sky room in the Valencia hotel is suposed to have great food. It is pricey, but the view is amazing.
As for Downtown SD, there is not a whole lot going on, but I really enjoyed Cucina Urbana - a modern Italian fusiony menu with a good wine bar on my last visit.
Make sure you hit up In N Out burger while you're there and get some good baja style fish tacos too.