A sikkk sound system, wiv loads of bass buckets! And one of those flashy multicolour interior lights for halfords.
On a serious note, scrap leds in the footwell. They're annoying as hell and look s**t.
I'd imagine the headunit has been changed, so maybe some decent speakers, nothing lairy, quality over quantity.
Bucket seats.
Steering wheel.
And as above nos! If you can't afford it then spray a milk bottle silver and wedge it under your seat with some speaker cable poked in through the foil top. The maccys lot will go nuts when they see it, and you'll have b**ches wet as f**k, slug trails and everything.