The town of Townsville isn't itself an Australian destination that I would strongly recommend. It's neither pretty nor interesting, nevertheless, it's a town which provides travellers with a pleasant atmosphere and a little less ostentatious than other Queensland cities such as Gold Coast or Sunshine Coast.
The town of Townsville offers numerous excursions for the Great Barrier Reef just opposite the town, and preferential access to the surroundings National Parks: Paluma Range National Park and Bowling Green Bay National Park. Additionally, you'll be directly opposite Magnetic Island, the incredible paradise island, where you will be able to go scuba diving, a flagship activity of this region of Australia.
Townsville is a fairly small town, which doesn't have any particular appeal during a trip to Australia. In spite of that, I recommend visiting the aquarium, which is large, with a big collection of marine species of all kinds. In addition, you'll get to see fish there which you will then have the opportunity to see on your Great Barrier Reef trips! Something which is really quite nice!
If you're staying in Townsville more than two days, go and spend a day on Magnetic Island, which is an island off the town's coast. It really is a region worth the detour; you can hire bikes or just simply go for long hikes on the island. Please be aware that it's fairly mountainous, so it's a little uneven! But once at height it's really worth it, as you'll have a view over the island, its forest and the ocean. This is why I recommend that you stay 2 days in Townsville: one day to visit the town and another to go to Magnetic Island.