Being over prepared rather than under prepared is the better situation every time. Having something you need and not using it is better than needing something and not having it. Always pack your ATV based upon that way of thinking. Taking into consideration the criteria we discussed in Trail Kit- The Intro on what your ride is going to be like, here is a basic trail kit one should have on any ride.
Tire Plugs
Air Pump or other inflation devices (such as C02 cartridges)
Rope or Tow Strap
Spare Spark Plug
Flashlight
Flat Head and Philips Head Screwdriver (or whatever your ATV uses)