Large Rectangular Touch Sensor
Or do number 4. Definitely the cheapest and simplest one. Make use of the resistance of your body. The average human got about 5MΩ resistance in their skin. Passing through a small current is absolutely okay. You're just sensing, not electrocuting.

Use a Darlington transistor (make one with two BJT transistors). As you can see, the \$V_{low}\$ is about 0.5V, in reality it will be closer to 0.7V. If that is too high for your µC(microController) to understand that it is a low, then add another transistor at the output which will work as a buffer.



Here's the link if you want to interact with the schematic. 



If the µC can't register that 0.7V is a low input. Then this schematic will guarantee that it will work.



Here's the link if you want to interact with the schematic.