if you wanted to justify the fact that your CV shows that you randomly change job every year or two by saying that you, y'know, liked new challenges and that?

I've already tried:

FEARLESS (too macho)
BRAVE (what you call a kid who cries at the dentist)
ENTHUSIASTIC (ie a moron)
UNRELIABLE (too close to home).

I know there's a word out there, but WHAT IS IT?