Restaurant wise, there's plenty of places to eat in cairns along the esplanade. Most offer 10 - 20% discount if you're seated early (before 6 or 7pm I think).
The Rattle and hum backpackers bar does decent pizza and from memory has $10 jugs of mega swill between 5 and 7.
Getting out to the reef can be very expensive (about $300 each), but day trips out to green and fitzroy Islands are quite affordable and great for families. Boats leave from the ferry terminal. You can buy tickets from the terminal, your hotel, or most backpackers (backpackers sometimes have discount tickets). You can snorkel at little reefs just off both of islands - no doubt not close to being as good as on the outer reef, but if your not a serious diver and want to have a look at some nice coral and fishies, it does the trick... And is about 1/5 of the price. Snorkel / mask hire is about $5. You can also take your own lunch out to the island with you to further reduce cost.
The lagoon at cairns is good for the kids if you're looking for a lazy day. Get yourself a spot on the lawn, take a rug to sit on, sneak over to rattle and hum for a cheeky beer... too easy.
Mossman gorge: great for a rainforest walk. The boardwalk was done up a few years ago
If you've got a car, get up to cape tribulation. Awesome. If you can, stay the night, or two.
Cairns has a lot of backpackers, and is often seen as a place to go to get to other places (Palm cove, port Douglas, Cape trib, reef, kirranda). But it's also a good destination itself.
Have fun. It's a great part of Australia.