Wow I didn't know there were controllers without a UART, or such a small embedded-oriented controller to not have I2C.
However, it does appear to have SPI and that should work just fine (MOSI, MISO, and SCK). Pretty simple to understand how SPI works. SCK clocks the data travelling on the other two lines. It is driven by one of the devices which is defined as the master. Data travels one way on MOSI and the opposite way on MISO. Simple binary data that you can interpret however you wish.
MOSI = Master Out/Slave In
MISO = Master In/Slave Out
That said...you might have problems dealing with distance. UART can use transceivers to increase drive strength, noise tolerance, and distance. I2C as well to a lesser degree. SPI...not so much because it is meant to communicate between devices on the same PCB. I2C as well but not quite as much. However, there are buttloads of UART controllers, CAN controllers, and I2C controllers (as well as separate transceivers, buffers, and amplifiers ICs to increase the drive strength of these protocals) that are interfaced to MCU's via SPI. So you can take that route to add that capability onto a lacking MCU...or just get a slightly larger MCU.