If you have a car, definitely head up to Port Douglas for at least a day – it's beautiful up there & offers great access to the Daintree (free) and the reef (not so free). They also have toad races at Ironbar every night at 8pm, which is a lot of fun.
As for Cairns, there's always a party at Gilligan's (the huge backpacker hostel). about 7 years ago
My favorite activity has to be a trip up to the Atherton Tablelands to explore some gorgeous waterfalls and lakes. If you have a car, this trip is free..otherwise you can do a day trip that will include lunch for under $100. Of course, you can't skip the reef while here in Cairns, but there are some great companies that will take you out to the reef and include an introductory dive.
For the best bar, I love Rhinobar. With great music and parties and different specials every night, Rhinobar is definitely the best party bar in Cairns.
For more great things to do in Cairns, just check out the MadMag Destiniation Guide as its got lots of suggestions for different activities in the area.
http://nomadshostels.com/Cairns/destination-cairns-cairns-travel-guide-a-advice about 7 years ago
Drive the coast road up to Port Douglas and then onto the Daintree. Its a fantastic drive and once you are Daintree there is a lot to see and do for free - the boardwalk through the forest is simply wonderful.
Grab a fishing rod and hit the pier at Palm Cove. The actual resort/place is a little expensive for my tastes, but take some food and do a BBQ at the beach and fish. Watch out for the pesky eagle (?) that swoops in and tries to get your fish..you have to be quick!
Cairns is definitely a city where you can do things cheaply - its a jumping off point for the GBR so there are always lots of backpackers and others around. The hostels are really good and there are lots of bars doing offers.
Agree with Angelina about the Atherton Tablelands - simply stunning. about 7 years ago
I would head out the the great barrier reef for diving out of cairns. cairns had a heap of dive tour operations and offer a wide range of day Trip and live aboard options for any traveler to the region. Some of the Better company's would be.
http://www.divedirectory.com.au - Discount Cairns Dive trips
http://www.diversden.com.au - Padi Dive Course's and Reef trips
http://www.taka.com.au - God Hole reef trips and Live-a boards
http://www.ocean-quest.com.au - Live-aboard reef trips and day trips almost 6 years ago