The adjective whimsical describes something or someone strange, unusual or playful in thoughts or action in an appealing way. 

Synonyms are weird, freakish, impulsive, or odd.

The word origins from whimsy plus -ical and was first used in 1653.

I love books with whimsical charms.

Once you get to know him, you will be surprised by his whimsical personality. 

It is just something whimsical I painted a long time ago.