Formal:
* Simple as ABC
* Straightforward
* Uncomplicated
* Effortless
* Child's play
* A piece of cake
* No sweat
* A breeze
Informal:
* No brainer
* Easy peasy
* As simple as that
* A walk in the park
* A cinch
* A snap
* Like shooting fish in a barrel
* Like taking candy from a baby
More descriptive:
* If the task is quick: As fast as a speeding bullet
* If the task is clear: Crystal clear
* If the task is obvious: As plain as day
* If the task is enjoyable: A joyride
* If the task requires little effort: As light as a feather
The best alternative will depend on your specific situation and audience.