Set Up
Every instrument can benefit from a professional setup.  A setup includes neck adjustment, action adjustment at nut and saddle, minor fret work, tightening of  screws and nuts, lubricate tuning machines, general clean up, re-stringing.  These adjustments will allow the instrument to play as well as it can.  If your instrument has never had a setup done, and most have not, you will notice a huge difference in the playability of the instrument. 
Upgrade to a Bone Nut
Most guitar manufacturers use some sort of plastic material for the nut.  All instruments will benefit from an upgrade.  Bone is considered the best material for clear crisp tones.  Consider this upgrade when your guiitar is in for setup.

Broken Head Stock!
This is actually a common injury.  In most cases, this can be repaired quite effectivley.  I will show you the steps here.  Fractured Head Repair
Fret Work
As frets wear, the tone will suffer and the instrument will become more and more difficult to play.  A simple fret dressing may solve the issues.  The fret tops are ground flat and then each fret is re-crowned and polished.  At some point however, as the wear becomes too extensive, the frets will have to be replaced. Check out somed detail shots here.  Fretting
Vintage Restoration Work
These old instruments deserve care and attention.  Check out this beautiful 1920's Stella.  Stella Restoration

Or this mid 1930's steel guitar.  Made in Canada by Charles Brasher.  Brasher Slide