I put steel sights on them right away, WELL worth the price and I consider it a mandatory change.
I do a LIGHT polish of the trigger mechanism, 25 cent triggerjob basically but not done very energetically. Basically I am just looking to get any big hitches out of it and get it pretty smooth. Trigger weight on a stock Glock is just fine for defensive or competitive use to me. I think this is worth the price since it basically costs nothing, if I was able to pick from 3 or 4 when I bought one I probably would just take the smooth one and not bother polishing anything.
I put skateboard tape on the grip. It costs less than a quarter and I can't shoot one nearly as well without it, so it is well worth the price. I think this is mandatory too.
That is all I will do to my Glocks, and all I will recommend that anyone else does. I shoot competitively and see Glocks malfunction constantly, the common thing between ALL of the Glocks I see malfunction is the owners have been piddling around with them way too much. They change springs, strikers, connectors, guide rods, all kinds of stuff. I can't even fathom spending a couple hundred bucks to make a great gun a lot less reliable. The people that just add ammo go a LONG time between malfunctions if they even have a malfunction.