Should I say: "I'll fly free to Honolulu" , "I'll fly for free to Honolulu", or "I'll fly to Honolulu for free"?
You could as well say, I'll fly to Honolulu free of charge.
You cannot have anything for free. You can have something free; you can have something for nothing.
I can fly to Honolulu free.
I can fly to Honolulu for nothing.
I can fly to Honolulu for no payment.
or, as Emgee says,
I can fly to Honolulu for no charge.
free = "without cost"
You do not need the "for"
It is free.