Good diction requires the crisp, clear pronunciation of consonants, without which the audience would be incapable of understanding a word you were singing.
Tongue twisters are good exercises for consonants such as

Peter Piper picked a peck of pickled peppers;
A peck of pickled peppers Peter Piper picked;
If Peter Piper picked a peck of pickled peppers,
Where’s the peck of pickled peppers Peter Piper picked?
…or…

Money making Monday morning makes a merry mind
(Repeat four times and increase the pitch by a semi tone (half note)
each time it is repeated
…or…
Ninety nine enchanting numbers nimble nymphs now dance
(Repeat four times and increase the pitch by a semi tone (half note)
each time it is repeated
…or…
The tip of the tongue and the teeth and the lips
(Repeat four times and increase the pitch by a semi tone (half note)
each time it is repeated

The consonant exercises are to be repeated increasing in speed each time, they’re fun and a great way to improve our diction.