I started playing with them back in 2011 ... I made a cool auxiliary dashboard system for my wife's car, it mostly posts time, date, outside temperature, some inside temperatures and compass heading

No actual Arduinos inside that one, it's all custom circuit boards I designed, but all pieces of the hardware were prototyped using Arduino and the firmware/code was written using the Arduino IDE platform.
There's a lot you can do with them. There's also a wide variety of them now, with different microcontrollers with varying capabilities and whatnot. I don't use the Atmel AVR arduino stuff anymore, I moved over to Texas Instruments' LaunchPad system back in 2012, which has a similar kind of thing going on. One thing I highly recommend looking into is Seeed Studio's "Grove" system - it's an ecosystem of sensors, displays, buttons, etc. that you can plug into a central "hub" of connectors (available as an Arduino shield or adapter for other development boards) that lets you prototype different things on the fly. They have starter kits with a bunch of sensors/etc. you can buy to get started.