As a bird:
* The lark soared high above the meadow, singing its cheerful song.
* We were awakened by the sound of a lark singing at dawn.
As a playful or lighthearted activity:
* We decided to go for a lark in the park, just enjoying the sunshine and each other's company.
* It was just a bit of a lark, a harmless prank we thought would be funny.
As a slang term for a person:
* He's a real lark, always up for a good time and making everyone laugh.
* She's a bit of a lark at times, but she's got a good heart.
As a verb meaning "to frolic":
* The children larked around in the playground, full of energy.
* The students were larking about in the library, causing quite a commotion.
These are just a few examples. The meaning of "lark" can be quite flexible, so the best way to use it depends on the context of your sentence.