Small gifts/presents that are meant to brighten someone's day. Often used when the gift is too small to really be deemed a "present", so it's just a little happy. Sometimes happies are given for no reason at all. Happies are different than gifts because there is usually no expectation that the recipient will reciprocate.
Commonly used in the southeast.
I baked brownies last night and brought them in as a happy for everyone at the office.
My best friend was feeling really down so I got her some little happies from the bakery.
My mom sent me some exam week happies - candy, hot chocolate and new lip gloss.