A drone
The toy that John figured out he could use to follow his wife was a professional drone that he had bought a year or two before all this began. It was a large drone that made some noise, but it could fly hundreds of meters up into the sky and be undetected.
This was perfect for spying on Donna, he could follow her car in the morning after she left for work and she would never see or hear it. Then he could see just what she was doing before and after work and if it was nothing, he could sleep easily at night.