All three, but not OTT.
Engine wise its not worth pouring dollars into. Just intake/exhaust maybe - get the air/gases flowing through high flow panel/pod filter, extractors, high flow cat and catback if you're after a little bit more go.
Exterior stock does look fine. As said, rims, lowered and its fine. Go a kit if you want it to look a little tougher or differentiate it from the other 34's getting around.
Interior - leatherette seats (seen them in group buys on here that looked alright), gauges, or as you said, sound system. I guess if you have some big subs in the boot weighing it down, you'll have excuse for the slowness of our NA's