General Alternatives:
* Logorrhea (refers to excessive talking, but can be used to describe a rapid sequence of difficult sounds)
* Verbal gymnastics
* Speech impediment (though not technically a synonym, it's often used to describe the difficulty of saying tongue twisters)
* Word puzzle
* Brain teaser
More Specific Alternatives:
* Alliteration: If the tongue twister is built on repeated initial consonant sounds (e.g., "Peter Piper picked a peck of pickled peppers")
* Phonological puzzle: This emphasizes the sound-based challenge of the tongue twister.
Ultimately, the best alternative depends on the specific context and the nuance you're trying to convey.