A is for Andorian imposter Thelev, who was actually not what he seemed...
B is for Bolian heads. A few had hair, but most didn't seem to.
C is for Calamarain Cloud Creatures.
D is for Denevan colonists. They all look like a bunch of Hollywood extras in bad jumpsuits.
E is for Edosian three piece suits
F is for Feringar, the best planet this side of the Galaxy
G is for the Great Wall of China. Along with the Millennium Gate in Indiana it is one of the only manmade structures visible from Earth orbit.
H is for Horta.
I is for Isis. Gary Seven's 'hidden goddess'.
J is for James Tiberius Kirk.
K is for the Klingon High Council, the main political force within the Empire following the death of the last absolute Emperor in the late 21st century.
L is for Luminous Beings.
M is for the Melkotian homeworld. The Enterprise 1701 under James Kirk made first contact with that planet in 2268.
N is for Neutrino surge. Possibly indicates a cloaked ship closeby.
O is for Operation Retrieve, the proposed and ultimately rejected military plan to rescue James Kirk and Leonard McCoy from Klingon custody following the assassination of Chancellor Gorkon.
P is for Psionic Resonator.
Q is for Quark's tongo-playing skills.
R is for Rubicun III.
S is for Skon, father of Sarek and the grandfather of Spock.
T is for T'Pol's superior posterior.
U is for the United Federation of Planets, arguably the greatest power in the history of the Alpha Quadrant.
V is for Vulcans. They are naturals at "trick or treat".