Carambola 2 has SPI and I2C protocols, but both of which are software simulated and can only work in master mode. USB is also a viable option, but neither I, nor anyone else could answer which is the best/easiest way as that is very skill-set and application dependent.
As for examples you could read
viewtopic.php?f=13&t=833,
viewtopic.php?f=13&t=782,
viewtopic.php?f=13&t=832, search for older posts and also on other (especially OpenWRT) forums.