General:
* Babble
* Chatter
* Gabble
* Jargon
* Natter
* Ramble
* Trivial talk
* Gush
* Waffle
More informal:
* Blah blah blah
* Gibberish
* Go on and on
* Talk a blue streak
More formal:
* Discourse (if the prattle is more about a specific subject)
* Harangue (if the prattle is long and tiresome)
* Loquacious (as an adjective describing someone who prattles)
The best synonym for "prattle" will depend on the specific situation and the tone you want to convey.