Comparing to objects:
* As sharp as a tack.
* As bright as a lightbulb.
* As quick as a whip.
* As clever as a fox.
* As smooth as silk. (This can refer to social intelligence)
Comparing to animals:
* As wise as an owl.
* As cunning as a snake.
Comparing to other abstract ideas:
* As insightful as a poem.
* As perceptive as a detective.
* As resourceful as a sailor.
The best simile to use will depend on the specific context and the type of intelligence you want to emphasize.