Parking sensors - I had rear only fitted to my W211 saloon, wondered how I'd managed without. Local car audio guy did a terrific job, they were so neat that they were hard to see, so better looks than OEM. I opted for just a buzzer, no visual display. Great job for touch over £200.
Seat heaters - I now have them and place no real value on them, so in my view, not worth the considerable expense and effort.