Yardening is not a precise activity requiring close tolerance in measuring what we are doing. At the same time having some idea of the parameters of something are often interesting and sometimes helpful in getting the job done more quickly and with more success.
Soil Thermometer - A VEGETETABLE GARDENER SHOULD NEVER PLANT SEEDS OR SEEDLINGS WITHOUT CHECKING THE SOIL TEMPERATURE TO DETERMINE IF THE TIME IS RIGHT. Plants don't grow much in temps over 80 degrees so knowing when that happens helps to understand why your plants are just sitting there.
Water meter - This tool is handy indoors or outdoors. It tells you when you plant needs water.
Light Meters - These devices are handy in trying to determine whether a spot in the yard has too much sun or not enough.
pH Meters - While the pH of the soil seldom changes unless you do something, this meter is handy to have if you are wondering about the pH of the soil in a particular part of the yard.
Soil Test Kit - These will give you a fairly accurate picture of the chemical content of your soil.