Bushwalk Australia

G'day I'm in Cairns for work that ends the end of September so could stick around a little while.

Is there a good overnight walk or place to see while I'm here? Can delay my return flight for a few days.
Ta

Can't help with overnighters but I've been to Cairns and Port Douglas a number of times with mrs.
So I've done a number of short day hikes etc around the place. I'm assuming you'll have your own transport?
In Cairns itself the botanic gardens and adjoining Mt Lumley Hill are worth checking out. Also the mangrove boardwalk near the airport.
Take a day trip on the ferry to Fitzroy Island. Go snorkelling with the sea turtles then do the circuit hike via Mt Fitzroy.
Going south Walshs Pyramid near Gordonvale just out of Cairns is a good challenge. Strenuous climb but really good views.
Keep going south to visit the Babinda Boulders, then Josephine Falls at the base of Mt Bartle Frere,
Climbing that could be an overnight option but I haven't gone up there and I believe it's a challenging walk.
Heading north from Cairns the walk to Hartleys Falls, near the crocodile farm, is good and few seem to go there.
The Bump Track near Port Douglas has some options. I've walked from the top end down to Mowbray Falls, which I can recommend.
Further on Mossman Gorge, Daintree NP and Cape Tribulation are all worth visiting.
Be mindful of crocs and cassowaries in some of these places, you will see warning signs at risky locations.
Inland we've been to the Atherton Tableland a few times. There are a number of short walks to waterfalls etc.
More challenging walks are there also but I've yet to explore those.

Like John, I get up to Cairns a bit.
I have not tried an overnighter, but I have climbed Bartle Frere [1500m climb] as a long day hike and you pass a shelter near the top that would assist for overnighting.
Humidity makes that a draining walk and I suffered cramping on the way down arriving back just on sunset - take a torch.
Take plenty of rehydrates and water.
But a faster walker can get up and back in good time. Starts at Josephine Falls.
Someone else wrote a trip report: https://bushwalk.com/forum/viewtopic.php?f=48&t=30347

My list of day walking:
Kahlpahlim Rock [Lamb Range] - a 4WD is useful to get to the start point
Walshs Pyramid - 900m up nearly from sea level.
Behana Gorge [6km return - nice swimming holes and falls]
Davies Creek Falls [as in the TV beer adds]
Emerald Creek Falls
Mt Baldy behind Atherton
Glacier Rock [6.4km return - near Barron Gorge]

If you are able to drive - Wallaman falls is the highest single drop in Australia.
With a 4WD - Blencoe Falls area is fantastic.
I also notice a new walk out of Cooktown has opened up that looks quite interesting over a few days. https://bushwalk.com/forum/viewtopic.php?f=38&t=37028
Note also https://bushwalk.com/forum/viewtopic.php?f=38&t=28376

Enjoy -
do not touch any heart shaped leaves [Stinging Tree],
always back out of "wait-a-while" vine [also known as Lawyers Vine] (if it is getting dull you will not even see them before you are jerked to a stop)
Keep clear of Cassowaries.