"הסינתיסייזר הוא כלִי נגינה מוזר."
Translation:The synthesizer is a strange instrument.
No, the theremin is a strange instrument. Synthesizers are relatively normal.
You could use the word "כלי" as a translation of the noun form of the word "implement", but I guess the word "כלי" would fit the word "tool" better. Though, you are not mistaken.
"By itself כלי is something like "implement," yes?"
Yes, and coupled with other words, it can "morph" into other words. כלי נגינה = musical instrument(s - depending on how you say it, this spelling can be singular or plural). כלי עבודה = tool(s). כלי בית is one you'll often see on storefronts - that refers to housewares. In a spiritual context, כלי refers to our abilities and attributes. I'm sure there are many more! כלי is an awesome, useful word.