Simplest way to send data to and from FPGA
USB is much too complicated
SPI and I2C can't be interfaced directly to your PC.

A serial interface (UART) is quite simple to implement in a FPGA.
You just need to add a level-converter (e.g. MAX3232 3.3V &harr; RS232) hardware.
In case your PC doesn't have a RS232 interface you can add an off-the-shelf RS232-USB converter (5 EUR) or use a ready-made 3.3V &harr; RS232 level-converter with integrated USB converter.