I may be able to elaborate, we commission 4-6 medium size plants and probably a dozen skids each year.
The mid size plants are DCS based, usually Foundation Fieldbus.
First few days you walk around just looking at things, typically you find lots of problems like missing instruments, instruments in backwards, instruments missing altogether.
You don't need a lot of hardware usually the DCS is able to communicate with the field instruments, if not chances are there-s some sort of wiring mistake.
These days you don't often see calibration equipment, the transmitters are more accurate than most test equipment.
Hopefully by the commissioning stage the client has assembled a team of instrument techs who are keen to learn, they supply all tools and test equipment.
For a non Foundation Fieldbus plant the transmitters are usually HART so you need a mead of communicating with them either by laptop or a HART configurator. A good multimeter e.g. Fluke 75. a 4-20 mA simulator.
An accurate gauge for setting pressure regulators.
We usually spend about a week checking all the trips
Followed by water batching which checks things like flowmeters, level transmitters, switches etc