Using nature:
* Pretty as a blooming rose
* Pretty as a summer day
* Pretty as a field of wildflowers
* Pretty as a sparkling stream
* Pretty as a sunset
Using other things:
* Pretty as a picture
* Pretty as a pearl
* Pretty as a snowflake
* Pretty as a polished gemstone
* Pretty as a melody
Using people or animals:
* Pretty as a princess
* Pretty as a ballerina
* Pretty as a kitten
* Pretty as a cherub
* Pretty as a doll
Humorous:
* Pretty as a pig in mud
* Pretty as a broken mirror
The best simile to use will depend on the context and what you want to emphasize.