* High-pitched
* Shrill
* Piercing
* Bright
* Sharp
* Tinkling
* Whimsical
You might also use words related to its role in an orchestra or band, like:
* Flute (though it's not quite the same)
* Woodwind
* Soloist (when the piccolo is featured)
Ultimately, the best synonym depends on the specific context you're using it in.