General:
* As goofy as a clown
* As goofy as a cartoon character
* As goofy as a rubber chicken
* As goofy as a bag of marbles
* As goofy as a playground slide
* As goofy as a sock puppet
Animal-based:
* As goofy as a puppy
* As goofy as a monkey
* As goofy as a penguin
* As goofy as a duck
* As goofy as a goose
Object-based:
* As goofy as a beach ball
* As goofy as a squeaky toy
* As goofy as a wig
* As goofy as a rubber band ball
Other:
* As goofy as a day at the beach
* As goofy as a summer day
* As goofy as a dream
The best simile for you will depend on the specific context and the level of goofiness you want to convey.