Take the ferry over to magnetic island for a couple of days when you're in Townsville, hire a mini moke there and take in the sights, or chill at the beach, surf, hire a boat, go to one of the bars there. Plenty of places on the island full of people your age.
It might also be worth going further north from Townsville if you have time, check out the Whitsundays and hit up Cairns, that place is also FULL of young travellers.
take a boat trip out to the Great Barrier Reef (maybe not from Townsville, it's not that close to the shore there.
Sydney is a really, really long way by car from Townsville, over 2000 kilometres. but if you do go there, check out Bondi, Coogee, and Manly beaches and obviously the Opera House. The nightlife in King's Cross is world famous if that's what you're into.
If you drive to Sydney, be sure to check out the Sunshine Coast (Noosa, Maroochydore, Caloundra etc.), Brisbane and the Gold Coast and Surfers Paradise, which are all big tourist spots. Go to some of the theme parks if you're interested.