My car has finally reached 160,000mi.
I want to thank my G with a nice good tune up.
Any suggestions or advice on where I should start?

2003 G35 Coupe 5at

 

All fluids: oil/trans/coolant/brake
Filters
Spark plugs
Belts
Brakes (depending on last change)

If they haven't been replaced already, I'd look into
Cam Position sensors
Valve cover/gasket

 

Should I also change my differential fluid?
If so, what fluid is recommended for that and trans also?

 

Yes change diff fluid also. Grab 2 quarts of 80W-90 GL5
Trans is auto, so you'd have to look up what type it takes and how much for a drain & fill.

VenomGT9 hit the major items. I would add:
AMSOIL engine flush for good measure
PCV valve
MAF & Throttle Body cleaners
Power Steering fluid (essentially all fluids in the car)
Upstream O2 sensors

 

If it's AT, use Nissan Matic S.