I second the interior LEDs. I got mine from @
DiodeDynamics and they are a little more expensive than the cheap ones on ebay but they are quality stuff, come with a year warranty if one fails early. It was probably the most noticeable early modification for me and it really makes the blue interior shine.
Another thought would be to get a knockoff "diffuser" for very little. I think a vendor here has them for $60 or $65 shipped. In my view it completes the look of the rear end and with the 2015s the stock tips are no longer the anemic ones on the previous years.