Some cheap sensors for your wireless Arduino node. Look for them on e*bay for the cheap price.
$2, Ultrasonic distance sensor.
Good for measuring distance of things, with resolutions down to an inch, and a range of between 3 inch to 6 feet (although I’m unsure about the upper range). May suffer from odd readings because of echo. I’m using this for the garage door monitoring Arduino.
$2, PIR (Infrared) Motion Sensor
I use these as touchless buttons to turn on lights. Also useful as part of a DIY security alarm. These have two pots to adjust sensitivity and duration of the output pin. Watch for variations – some can work at 3.3V, some have to have 5V or more. You may be able to cut the voltage regulator and power it directly from a 3.3V system. Remember the pull-up resistor.
I use it in the garage to see if the light got left on.
$6, Smoke & combustible gas sensor MQ-2
Email yourself when your house is on fire.