Will I be cold in a sleeveless dress at night? Can we wear sandals? Or should I take this all to mean I should pack as if it’s a Canadian fall?
We're usually there some time during the first two weeks of December. We try to minimize our packing as much as possible while accounting for unpredictable and wide swings of temperatures. While there may be some days, or portions of days, that are warm enough for shorts or dresses, I don't think you get enough use out of them to justify the luggage space. So we usually take only long pants in a mix of lightweight and slightly heavier khakis or similar, no shorts or dresses, and a mix of long and short sleeved shirts with some (but not all) of the long sleeved ones being slightly heavier material. I travel in a zip-up fleece/thermal jacket (I often use an Icebreaker mid-weight merino wool jacket), a rain jacket and jeans so as not to have to pack them yet have them available if necessary. I never bring a winter hat (I do bring a Tilley sun hat) but I always have lightweight gloves in my rain jacket pockets. I like the zip-up fleece as opposed to a sweatshirt since I can leave it open for a bit of air flow, zip it up for more warmth or take it off and tie the arms around my waist or purse strap if it gets really hot unexpectedly. That pretty much covers all possibilities - short sleeve, long sleeved, heavy material, lightweight materials, waterproof, thermal, and I can mix all of them depending on the weather any day plus being warm enough when leaving and returning home. I wear good walking shoes to travel and tour in - no additional luggage space required for shoes. I only bring sandals if I've brought a fancier dress to go to Victoria & Albert's; I would personally never be comfortable to walk around for hours in sandals regardless of the temperature but if the temperatures drop, you will definitely wish you had closed shoes, especially if it rains and is cold.
Some days will be 30 or so degrees Celsius but only for a few hours during the day, colder early in the morning and very chilly after dark. Sometimes it can be 30 during the day and not much cooler at night but still not really shorts weather. Other times it can drop to 10 degrees even though it was 30 earlier in the day. We've been some times when it was about 15 degrees one day and then 28-30 the next day. It's just very very changeable and difficult to anticipate ahead of time. This is our first year going between Christmas and New Year's so I'm still figuring out myself how to modify my usual packing for December.
I’m usually packing tonnes of sunscreen. Now I’m thinking we won’t even need it .
You will still need it. Even if it's colder, it will still likely be sunny and you can still get burned. You might also have warm days where you might be wearing shorter sleeves and/or be in the pool. The pools are nicely heated (and there are hot tubs too). We wear swim shirts - I sometimes wear a long sleeved swim shirt over a bathing suit tank top and capri or full-length swim pants - both for warmth and to protect from the sun. While I would never travel without sunscreen, a proper UV-protective swim shirt is much easier than ensuring you get full coverage with sunscreen and you don't need to reapply afterwards. It also helps to withstand the wind when getting out of the pool. A couple of years ago, we went back to our resort pool by about 1 pm because we were just dripping sweat. The next day we decided to go back to the pool. There was about a 15 degree downwards swing between the two days and we were only able to withstand about 45 minutes and that only because our swim shirts helped keep us a bit warm. Both days very bright and very sunny. Definitely needed sunscreen on neck and face.
