Is Arduino ideal for making a HRNG (hardware random number generator)?
I think you'll want to go down the path busz suggests. Search for the concept "diode noise". The PN junctions in diodes and transistors can produce close to perfect Gaussian white noise.  Sampling that should be a source of entropy that's better than any environmental source.  

The problem with most environmental/ambient data is the values just don't change that much over time: temperature, humidity, light and sound all have less than an order-of-magnitude of variability with really strong modes. An accelerometer to measure motion might be a good source of variability if mounted on a person, but you'd likely have to do a bit of signal processing to remove the normal modes of oscillation that are present in how humans move.  An ambient light &amp; sound source might have some pretty high variability if placed in an high-density urban space, but again I think there would be a lot of repetition.  I still think the best source of entropy would be going down towards fundamental physical properties of materials like diode noise than going up in scale and looking at environmental factors capable of being read by a microcontroller.