What's the simplest way to get arduino functionality?
If you what you're looking for is the cheapest option with the fewer components possible, then you can use a regular ATMega328p chip, with a few passive components attached to it.

You just need to program it with an Arduino Bootloader once, in order to be able to program it easily, but you can use any existing Arduino to do that. After doing this once, you can program it with any FTDI-type USB to Serial converter.

Here's an Instructable that explains the whole process.