WARNING: Spoiler-heavy answer
There are exactly 25 new upgrades, nine of which are cookies, and 6 new achievements. There are also the new reindeer and Santa game mechanics.
To start everything off, you need to purchase an upgrade called "A festive hat," which costs 25 cookies. This will unlock an icon below your cookie.
The icon reflects Santa's level and starts at "Festive test tube." It can be upgraded for 1 cookie to reach the next level. The next level will cost 4 (22) cookies, then 27 (33), up to its final level, which will cost 1414, a little over 11 quadrillion cookies.
Whenever you upgrade Santa's level (including the time you purchase the festive hat), you will unlock a random upgrade, which cost 2,525 cookies each:
- Increased merriness - Cookie production multiplier +15%
- Improved jolliness - Cookie production multiplier +15%
- A lump of coal - Cookie production multiplier +1%
- An itchy sweater - Cookie production multiplier +1%
- Reindeer baking grounds - Reindeer appear twice as frequently
- Weighted sleighs - Reindeer are twice as slow
- Ho ho ho-flavored frosting - Reindeer give twice as much
- Season savings - All buildings are 1% cheaper
- Toy workshop - All upgrades are 5% cheaper
- Naughty list - Grandmas are twice as productive
- Santa's bottomless bag - Random drops are 10% more common1
- Santa's helpers - Clicking is 10% more powerful
- Santa's legacy - Cookie production multiplier +10% per Santa's levels2
- Santa's milk and cookies - Milk is 5% more powerful
A final upgrade costing 2,525,252,525,252,525 cookies is then unlocked once Santa reaches his final form:
- Santa's dominion - Cookie production multiplier +50%. All buildings are 1% cheaper. All upgrades are 2% cheaper.
1 Christmas-themed cookie drops from reindeer
2 Stacked in additive fashion, eg: +10%, +20%, etc.
There are also reindeer. They work like golden cookies (and a bit like Wrinklers), in that they will appear at random intervals3, hopping across the screen. You then have to click them before they disappear. When you do that, they will give give you one minute's worth of cookie production4 (CPS*300) or 25 cookies, whichever is highest.
Clicking on reindeer also has a 20% chance5 of unlocking one of the seven Christmas-themed cookies. Each of them costs 252,525,252,525 cookies and give you a +20% cookie production multiplier. Note that similar to Halloween-themed cookies, if you already have the cookie it will not unlock a new one. This makes them harder to get the more you have.
There are also two new cookies ("Gingerbread men" and "Gingerbread trees") that automatically unlock once you've earned 99,999,999,999,999 cookies. They cost 9,999,999,999,999,999 (10 quadrillion) cookies each and give you a +25% cookie production multiplier.
3 Between three and six minutes; twice as frequently if you have the "Reindeer baking grounds" upgrade
4 Stackable with frenzies (ie: popping a reindeer during a frenzy will give you more cookies)
5 If you have the "Let it snow" achievement, it's a 40% chance. This means if you reset the game it will be easier to unlock them.
There are six new achievements. All of them are regular achievements, which means they give milk.
- Coming to town: Reach Santa's 7th form
- All heil Santa: Reach Santa's final form
- Let it snow: Unlock every Christmas-themed cookie
- Oh deer: Pop 1 reindeer
- Sleigh of hand: Pop 50 reindeer
- Reindeer sleigher: Pop 200 reindeer