Interior

I have always liked to know what my engine is doing and the legacy has less gauges than any other car I have owned, so I added a few - there are not many convenient places to add gauges to a legacy but I've tried.

Boost Gauge, in drivers side air vent - air vent still works
Voltmeter, Clock (Imported Legacies don't have clocks) and Oil pressure, these are all bought to blend in as well as possible with green backlighting and red/orange needles
I have replaced the standard (electric) front seats with the fronts seats from an Impreza STI.  These give much more lateral grip than the standard seats
Climate Control (very useful, set the desired temperature, and it gets on with everything else automatically - starts by demisting the screen , then warming the feet or cooling generally depending on the external/internal temperatures - adjusts direction, speed and temperature of airflow automatically and switches Air conditioning on and off as needed
Centre console, painted with aluminium paint