* As bright as the sun.
* As happy as a clam.
* Like a songbird in the morning.
* As lively as a spring breeze.
* As bubbly as champagne.
* As radiant as a sunflower.
* As light as a feather.
* As sweet as honey.
* As warm as a summer day.
* Like a child at play.
The best simile will depend on the specific context you are using it in.