We've been in January a few times (including our honeymoon!). Just check the weather forecast 2 or 3 days before you go. weather.com gives a 10 day forecast. I checked last year, and it really helped with packing.
Along with the pants/jeans and long sleeved shirts, make sure that you include one pair of shorts (you probably won't need more than one), a few short sleeved shirts, and a light jacket/pullover. Some days, you might find that you need the pullover in the morning, and you can take it off and just wear the short sleeved shirt in the afternoon. And don't forget a swimsuit. The pools are heated, and the deluxes and mods have hot tubs.
Have a great trip!