Australia, New Zealand & the South Pacific - Perth, Adelaide, and Melbourne times

I got a option of either including Adelaide or not in a trip.
18th I get into Perth about 6pm. 22nd I need to fly out of Melbourne back to the USA
I can either do 18th-20th in Perth, 20-21st in Adelaide then 21st-22nd in Melbourne or
18th to 20th very late in Perth then to Melbourne for the 20-22nd
or
18th-21st in Perth then overnighting in Melbourne.

What should I do? I was already once in Melbourne. OK city. Saw some sites.

What month?

18th-21st in Perth then overnighting in Melbourne gets my vote. Enough to see & do Perth - Freemantle & environs for 3 - 4 days. Otherwise you be spending many hours getting to airports, waiting in airport to board aircraft, in aircraft and getting from airports. PER-MEL is a 4 hour flight (and a 2 hour time change)

July.

So PER for 18-21st is a good option, but the hotels there are quite expensive. I can do 2 nights in the IC (BOGO cert) but don't know about the 3rd night. Could do the IC for the 3rd night but if there are other options I might go with that.

OK what time should I get into MEL?
I can dept PER 915 arr MEL 1440
or
PER 1140 MEL 1655
PER 1350 MEL 1915
PER 1525 MEL 2050

I dept the following day about 1120AM ON UA
If I grab the earlier one, I can stay in the CP MEL and grab the skybus (guessing best option for a affordable price) into town and grab dinner and walk around.
Or should I just take a later flight out of PER and stay at the airport hotel?