[AnimalMuppet]

Maybe ask your boss? "Yo, boss, I've got some spare time. What's the most valuable thing I can do with it?"

[UncleOxidant]

Careful with that. If you tell some bosses that you've got some spare time they'll just give you other tasks to do.

[Kipt_]

I echo this. Every time I've had nothing to do I mention it to the "boss" and the response has been: "Let's fix this".

And letting others know that you want to do work, is being proactive. At least according to my standards.