What would it be? It can be whatever you want, but only one thing can be changed!
I would swap out the 5 speed in my Outback for a 6 speed from an STI. I do a ton of highway driving so I feel as though the extra gear would boost my gas mileage and make cruising at 85mph on long trips a little bit easier on the engine.
What would you guys/gals do to your daily driver? Money is not a limiting factor, so go wild!