I've often thought about what the quintessential "blink the LED" project would be for IoT sensors. For my own personal journey, the first thing that I did was create a temperature and humidity sensor. My approach for the last year now has been to look for interesting sensors, purchase a few of them and then figure out how to get them connected.
My two goto places for interesting sensors has been Tindie and eBay. I still remember my first Tindie Purchase. It was a great feeling to get the board, solder it up and see the first few days of temp and humidity data streaming in.
In all I've sold 82 units to people all over the world. I've also given away quite a few to people looking for projects to get started with IoT. The big attractor for sales has been the blog post that I wrote for the Electric Imp community blog last October.
I even sent one to this guy:
Another great project for temperature only sensing was published here: http://www.instructables.com/id/TempBug-internet-connected-thermometer/
It is pretty interesting to now see the number of TempBugs that are now connected to analog.io! Last time I counted there were 18! I talk to people from all over about how they are using these sensors, in their homes, cabins, attics and basements. There has also been interest in deploying them in reptile tanks and cigar humidors.
So anyway, I do believe that Measuring the Temperature is something that every human can feel and empathize with, if you are looking for the first IoT project, I highly recommend trying one of these out.