What instruments or avionics do balloons usually have?
The magic regulation for minimum equipment on a balloon (in the USA) is 14CFR31.85 ("Required Basic Equipment" for manned free-air balloons).  

The FAA requires that balloons be equipped with:

For all balloons


  
  [Reserved]
  An altimeter.
  A rate of climb indicator.
  


For hot air balloons (additional equipment):


  
  A fuel quantity gauge.  
  An envelope temperature indicator.
  


For captive gas balloons (additional equipment):


  
  A compass.
  




Beyond what's required by regulation, in most free balloon operations I've seen they also carry an aviation radio and/or a CB radio (to communicate with their ground crew) - this is usually a handheld transceiver.

The FAA Balloon Flying Handbook also makes for interesting reading. They talk about required and optional instruments and equipment found on various types of balloons in Chapter 2.