"It is as bright as it could" is not a complete sentence. To make it a complete sentence, it would have to say "It is as bright as it could be". Furthermore, it is very difficult to rhyme with an entire sentence, and usually isn't necessary. Usually it is only necessary to rhyme with the last word in the sentence.
Bright, as in bright eyes.
Bright, as in a bright smile?
"A bright sunrise" could rhyme with "outside" and reference a good morning.
bright Tangier
star bright
sunny bunny
beef stew, bright blue?
insight? bright?
bright light
Take flight.
the rhymeing word is bright
bright light