CCSS.Math.2.OA.2 Fluently add and subtract within 20 using mental strategies.
I find, once the kids catch onto the tens trick, the nines trick is easy! I first use a number grid like THIS ONE to show them how easy it is to do "ten more". From there, it's pretty easy to figure out "nine more".
Once they have a good understanding of the 9 more trick, they're ready to play Turkey Nines! It works like "Old Maid", so they pair off all the addends and sums with nines, and one of your little turkeys will end up being the "Thanksgiving Turkey".
This is one of those games that "even the losers are winners" because there's something about cooking a turkey that makes the little ones giggle. Plus, they're getting better at that mental math!
When they've mastered the plus nines up through 20, there's a more challenging version of the game to work this standard:
CCSS.Math.Content.2.NBT.B.8 Mentally add 10 or 100 to a given number 100–900, and mentally subtract 10 or 100 from a given number 100–900.