One curious type of sensor that can be realized with simple means is a soil moisture sensor to monitor wether plants need to be watered. A simple way of achieving this is to measure the soil resistivity with two electrodes. Unfortunately that is not very reliable due to corrosion of the electrodes.

A much better approach is a capacitive sensor, for example as shown here. The sensing principle exploits the fact that water has a much higher (~80) relative dielectric constant than dry sol or air (~1). The sensing element consists of a planar capacitor that is directly inserted into the soil. This can be very easily realized by copper traces on a PCB that is insulated with solder resist. Coincidentally I found a sensing element that I designed years ago. The layout is shown here:

Below you can see how it is inserted into a potted plant.

Since this sensor is essentially a variable capacitor, it can be easily read out with the RFC peripheral. Since the capacitance of the sensor is relatively low, I chose a resistor of 47 kOhm to achieved a reasonably low frequency.

Of course, the resistor does also directly influence current consumption of the oscillator, hence choosing a larger resistor is sensible.
The table above shows counts for a counting time of 40ms. The sensor capacitance vs. air is only about twice as high as the parasitic capacitance on the breadboard ("no sensor"). The counts strongly decrease in presence of water and its easy to distinguish a freshly watered plant from one that already dried up slightly.
One bonus experiment to verify that the sensing principle is indeed capacitive: I deposited a drop of isopropyl alcohol on the sensor and monitored counts over time as shown above. Isopropyl alcohol has a dielectric constant between that of air and water and is well isolating. Accordingly, the counts sharply drop after application of the drop due to an increase of sensor capacitance. The alcohol slowly evaporates which leads to a steady increase of counts over time proportional to the loss of volume.

In summary, the RFC seems to be well suited for this application. Time for a design project?