Cairns Great Barrier Reef

How long will you be in Cairns? I think you should stay in Cairns long enough to go up to the rain forest as well. I would recommend two nights minimum. Wait till you see all the bats at sunset. You will be amazed.

I could spend two nights in Cairns or I could book it back to Sydney after just one. I loved (and am creeped out!) by the pics of the bats I've seen, so I was planning on checking them out the evening that I arrive in town. The rain forest could be interesting, I'll see what I can find out online.
Thanks for your help!

If you're referring to the flying foxes that used to live in the Royal Botanic Garden in Sydney be advised that the colony was moved a couple of years ago.
There are supposed to be a number of colonies in and around Cairns however. Am sure you can do a Google search to find the one nearest to your accommodation.
A visit to Kuranda would indeed be memorable, as would an excursion up to Daintree National Park... probably best done as part of a day tour.

Definitely stay the two nights. I stayed three nights and could have stayed longer. In Kuranda we visited the Koala Gardens and ate lunch at Frogs Restaurant. We then tookthe Skyway over the rain forest which was well worth it. What I will always remember are the bats. Their wingspans were up to 6 feet long. Here is a great video that I found in YouTube that was shot by our hotel and is exactly what we experienced. I would estimate that we saw over 10,000 bats during a 2 1/2 hour period at sunset from our balcony. Take a look at this video. I will remember this as much as I did The Great Barrier Reef.

https://youtu.be/a_MEDB2ilCs

Confused....is your tour to the GBR departing from Sidney? Or are you spending the night before in Cairns? It has been 10 years since I did a snorkeling day on the reef, which was fabulous. About 20 of us on a large cat, took about 1.5 hours, leaving at 8.00 am to reach the atoll - only one other boat there, a private yacht. Be aware there are two sorts of GBR tours, small ones like mine or the large party boats, who all seem to congregate around one or two islands. About half an hour for a safety talk and instruction for those who had not snorkeled before, we swam for a couple of hours then took a lunch break, and swam some more until it was time to return. Did not put my damp T-shirt on after lunch, big mistake...got sunburnt, luckily not badly! So you get back to Cairns around 4.00, hopefully you can grab a shower somewhere at the docks/marina, then you have to get out to the airport which is outside of town, so say 3 hours with security clearance, plus 3 hour flight time to Sydney,, plus time to get back to where you are staying. So you are back in Sydney at around 11pm. I hardly think you will be ready to go out on the town after such a day! I’d much rather spend another night in Cairns. It’s a tourist oriented place so lots to do in the evening...good restaurants, pubs etc. Very early one morning I took a hot air balloon ride from Kuranda, stunning as the sun came up, and rode the tramway back down. Didn’t make it to Daintree, but wish I had. Visit the large mangrove swamp between the city and the airport, eerie, hot, intriguing, scary, home to various species of snakes and crocs.

I second the replies that tell you to visit Kuranda. It's wonderful.