In a rough Order of usefulness & difficulty and need for tools:
Fix puncture
Split and fix a chain leaving no stiff links. (No power links!)
Adjust/set up V brakes.
Service a headset.
Pull cranks.
Replace chain rings.
Set up front mech.
Set up Canti brakes
Set up Rear mech
Recable bike.
Re-lube hubs and adjust cones.
Bleed discs/magura's
Fit headset cups and crown race.