Well, if you're looking for a really simple way to develop a level sensor, and I am understanding your definition of "level sensor" as something that will ultimately tell you how much water you have; you can use a potentiometer and a float system to provide a variable resistance output based upon the water level. This system was used in older model fuel tanks for automobiles and light aircraft to provide fuel level readings. Other similar contraptions that use the same float methodology can be viewed in your toilet tank, except this system is purely mechanical, but the idea is the same.
If you decide to go with the PCB system for detecting rain, I suggest placing the sensor at an angle, approximately 30-45 degrees to allow the water to run off of the PCB.