AEN filter. I understand it doesn't actually give an HP boost. I just like the fact that they are reusable. I may be trying other brands over the next years or so, and decided to try this one first since i heard it was a good deal.
AEM Dryflow Panel Air Filter - 2015 WRX / 2015 STI - Intake - 2015+ Subaru WRX
LED corner lights. I have a premium so i have those ugly orange ones. These are pretty damn cheap so i went with them and i think they will help make the car look better.
All in all, not bad for $60 or so (including shipping).