As above, have a quick look at the guide - You can get after market apps on the headunit with a little jiggling.
XA4's thread is probably the best inspiration you'll get!
For modifications, there's K&N Panel filters about for all models now AFAIK
, which is always a good start for a little added vroom noise; I've just installed Acuity shifter bushings which have markedly improved feel and function of the shifter too.
Lighting wise, some OSRAM T10 LED bulbs for the front map lights, dome light, boot light and numberplate lights are a good start (6 Needed in total!)
From there, LED Reversing lights are another easy improvement.
If you don't mind MOT Implications, you can fit HID bulbs into the projectors without issue (No blinding as they're projectors) - HIDs also work nicely in high beams if you are willing to sacrifice a second or so warmup time - They focus well with the reflectors unlike LEDs, and greatly increase light on the road VS halogens.
Inside, door sill plates if you haven't got the optional extras; A skunk2 Gearknob also looks nice and makes shifting even more effortless.
As for aftermarket wheels, I'm never quite sure - The original Argons I've got look a perfect fit for the car, but once fitted Rotas etc. do have a nice aesthetic: