A is for Art. Commander Data enjoyed painting during his time aboard the Enterprises-D and -E.
B is for Beagle. Captain Archer enjoyed playing with his Beagle, Porthos, in his spare time. Just don't feed him any cheese.
C is for Cardassians. They spent much of the 24th century in direct or indirect conflict with the Federation and other powers.
D is for Dominion, the rulers of the Gamma Quadrant.