I've had my S5 for 2 weeks now (well 2 weeks tomorrow) so I'm gradually getting used to the option I added:
Flat Bottomed Wheel - a good cheap improvement (although not heated which is a shame)
Red Brake Callipers - relatively cheap and really adds to the sporty look (against my mythos black paint)
Carbon Atlas Trim - very expensive for what it is, but really adds to the cabin quality and much better IMHO than the brushed aluminium (as the various controls along the strip blend in much better)
Light and Vision Pack - Matrix headlights and scrolling front indicators are great and the virtual cockpit is 100% a must get and comes as part of the pack. Extended interior lights (different colours) is nice for the first few minutes - then you settle on a preferred colour and may not use it again)
Sound and Comfort - Band and Olufsen is fantastic, Advanced key really handy as the keys stay in my pocket now, rear camera is useful for parking too as is hill hold assist (although think that now comes as standard)
Magma Red Seats - free and really really love them - although was unsure I'd made the right decision before delivery
Adaptive Suspension - had this on my 4 series BMW and like the fact you can alter the ride instantly
Panoramic Roof - really like this too, although £1250 is very steep
Tinted Rear windows - think it looks much 'meaner' with these!
Folding Mirrors - why is this not standard on a £50k car?
Didn't get but would if I had the money......
HUD - had this on my 435D M Sport but with the Virtual Cockpit can definitely live without it
Storage Pack - wish I had this as there are no hooks in the boot for shopping / curries etc
Tech Pack - if only for the front sensors - which look like fog lights and IMHO make the front look better.
TBH there's nothing I regret adding and nothing that I think I missed off that I'll really miss - and I love the car!!!