A nice touch is how the power relay handles heavier loads without making setup feel complicated.
GE MAGNETIC CONTACTOR CR120B 020 NEMA A600 P300 55-201501P001 110/120V 50/60HZ
Because it is black, the relay looks uniform when you group several units together in a row.
A nice touch from Miniduino is the small guide card that helps you get started with general purpose relays.