One great thing about arduino is the Amount of tutorials and libraries written for it. It seems that the list seems endless. We have started most commonly used stuff, and it has been a great fun and learning. You can track the progress on page below:

https://www.exploreembedded.com/wiki/Explore_M3_Arduino_Libraries

Here's a simple touch sensor!

The go to temperature, humidity sensor. Not quite accurate but does the job.

And the than the good old MQ sensors:

The ADXL335

And many more coming soon!