We've gone in December for the past few years...and we've done the first week, second and third week of December. I've never brought shorts with us in December. I don't think it's ever been hot enough for us to wear shorts.
It helps that we've had our winter coats with us...it's helped on occasion when it's gotten darn cold (at one point, I swear it was warmer at home.)
I bring what I call my "hotel slippers". They're nice comfy slippers that I only wear in hotels (never at home!). I bought a pair in San Franscisco's Chinatown in 93 and it lasted me 12 years!! I had to buy a new pair last year.
I always make sure I have 2 longsleeve shirts with me that I wear on the plane going and coming back. T-shirts the rest of the time. Sometimes I wear a T-shirt and one of the longsleeve shirts on top instead of a sweater or a sweatshirt...and then I can still wear the longsleeve shirt by itself later.
We've stayed at a timeshare the past couple times so we do laundry in the middle of the week. We don't have to overpack then (but we still do!)
My essential list (leaving out the toiletries and electronics)
- 2 longsleeve shirts
- 1 sweatshirt
- slippers
- enough underwear to last the week (cause even though I know I can wash 'em....I don't want to risk not having fresh ones!!!)
- 1 pair of pants/jeans (I'm wearing the other pair already)
- t-shirts to last the week (1/2 a week if I have access to a washing machine)
- comfy pjs (usually my pjs consist of a t-shirt and pj pants...the t-shirt can easily be converted to daytime use if there's an urgent need.)
- extra pair of shoes (nothing can ruin a vacation more than soggy shoes)
- 1 light jacket
- 1 winter jacket or a not so light jacket (this is what we wear to the airport in Toronto...we get to the airport and stuff it into our checked bags....it's there if we need it)
- lots of socks (don't be stingy on packing anything to do with your feet)
- bandaids
- cheapo umbrella (Dollar store umbrella works and if you lose it, it doesn't matter!)
- extra ziplock bags of differing sizes and small cable ties
These days, I pack my clothes in either ziplock bags or the Spacesaver bags. I find that I can pack more into the suitcase, avoid having airport security touch my clothes with gloves that's touched everybody else's things, and keep my clothes smelling as fresh as they were when I took them out of the laundry. Also lets me put my clothes into the drawers without worrying what was in the drawers before I got there. (gee...am I paranoid or what???)
And always leave room for souvenirs for the trip back.

I've started pin collecting...it's easier to pack pins into the void spaces of your luggage than t-shirts.