Mackay Regional Botanic Gardens sit beside the lagoons west of the centre and focus on plants from the Mackay region. That local angle matters: this is not just a lawn-and-flowerbed city garden, but a good introduction to the coast, rainforest edges, and dry tropical country around town.
The gardens are best in the cooler parts of the day. Bring water, and use the visitor centre if you want help choosing the shorter loops.