In no particular order, these are my top 5.
1. OEM+ upgrades, some higher spec, or international spec models have nice bumpers, kits, lights that work well for subtle mods
2. Handing (Suspension/Tyres/wheels) : makes the car nicer to drive, safer and look better
3. Performance: Minor upgrades to intake and exhaust, just to get a bit extra performance. A good service also helps.
4. ICE: I have only owned older cars, which have had pretty dismal stereos stock. A nice modern headunit and decent front speakers makes a good start to improving aural experience while driving
5. Security: Alarm to deter amateur theives, and dark tints to stop people looking in the car while parked, also looks good.