Sensor types/ranges? How to detect a pea-sized sample of some material, hidden behind a thin piece of wood, up to 2 feet away, hidden behind a thin piece of wood?

I'd say this is far to difficult a task to start with. The usual distance sensor technologies would be infrared, ultrasonic, and laser. None of them will go through a piece of wood. If your sample is magnetic, a hall effect sensor or other mag field sensor might work but the distance is I think long for most of those technologies. The place to start is in the science, what can you irradiate the sample with that will cause a detectable change through wood at that distance (I don't know of anything, but there might be). However once you figure that out you are likely going to discover that the solution has noise issues (i.e the signal is buried in noise) and requires a lot of very complex signal processing to recover it not a beginners project in short. I'd suggest you perhaps start out with one of the wheeled robot kits that either follows lines with infrared sensor or avoids walls with ultrasonic sensors to gain experience in both electronics and sensors, in other words start smaller Image may be NSFW.
Clik here to view.