there are a few threads about this
What's Ibiza like for New Years Eve?
NYE: the only of the big clubs open is pacha normally.
in the past, space has opened a few times and amnesia did two years ago. haven't heard anything for this year yet and if I had to guess I'd say that it'll be only pacha again. apart from that, smaller venues are open; underground for example, but also places like swag (doing electronic stuff in winter often), then sushipoint, bubbles, veto.
NYD: dc10 of course.
if you've never been to ibiza off-season it'll feel pretty weird at first because it's really quiet. pdb is a ghost town and much of san an is very quiet too. I'd recommend staying in ibiza town which stays a bit lively. hire a car and drive around the countryside - you wouldn't believe how green everything is here during winter.
temperatures can reach 20°C at lunchtime if the sun is out (very often the case), but it gets very chilly right after sunset. sunset is early in december (5pm) so you'll need layers. nighttime temperatures seldomly go below 5°C, but because of the humidity it still does feel pretty cold.
also - if you really consider coming down for NYE, pre-book a table for dinner somewhere for NYE. in spain, people first go for dinnner and people actually stay in the restaurants right through midnight (they eat the grapes at 12 o' clock - google it) and then only leave the restaurants at 1-2am to make their way to pacha for a 2-3am arrival (just like in summer really).