QF have a global sale on at the moment which I believe ends tomorrow Thu 09 May (need to book and pay by Thu 09 May) with fares in November of AUD1099.00 MEL/HNL return or AUD1249.00 MEL/LAX return. If you wanted to take QF93 MEL/LAX then QF4/QF489 HNL/xSYD/MEL for AUD1174.00 I'm sure you could have an open jaw itinerary if you wanted to stopover in Hawaii on the return but you'd need to call QF to book your flights on an open jaw basis as you can't do this on the QF website like a simple MEL/LAX/MEL airfare.
www.qantas.com.au
From either HNL, LAX or LAS you can purchase an instant upgrade fare to PTY (Panama City) on AA which means you pay an economy airfare but travel in first within the USA then Business ex mainland USA to Caribbean or Central America eg MIA/PTY.
The one way fare on AA LAX/PTY is USD615.45 per person connecting same day via MIA or you can have an overnight transit for USD625.45 as long as you don't exceed 24 hours in MIA. Stopovers longer than that are not permitted on a through airfare and it's generally not financially viable to pay for sector fares LAX/MIA then MIA/PTY.
If you wanted to have a couple of nights in Vegas you could purchase a cheap one way fare LAX/LAS then buy the fare LAS/PTY USD655.45 with a same day connection or USD701.15 having an overnight transit in MIA. Visit
www.aa.com and select US as your home country not Australia.
For PTY/HAV (Havana) return Copa Airlines (CM) have fares for USD592.50 return
www.copaair.com and they're a member of the Star Alliance.
Copa do not display through airfares LAX/xPTY/HAV nor do other US based booking engines such as
www.priceline.com however these fares will display elsewhere such as
www.jetabroad.com.au for around AUD1138.00 per person return.
Make sure that if you do snap up a QF sale fare by tomorrow that you take out travel insurance at the same time so you're covered for cancellation penalties due to unforeseen circumstances plus medical expenses.
Note that you will be in the USA over thanksgiving which is one of the busiest times of the year for air travel so you'll want to lock in your dates fairly soon.
Let us know what you finally end up doing.