Nestled away on the top floor of the Market City shopping centre in Haymarket are bumper cars for the kids. The cost is $5/car or you can get 4 rides on the dodgems for $15.
There's a sign saying height and weight restrictions apply so I'm pretty sure these dodgem cars are only for the kids but they'd make a great surprise for the kids if you've taken them into the city for a day of shopping or whatever. Dodgems are a great reward for behaving for the day!
Walk into the market city centre, catch the lift to the top floor and walk to the back of the amusements centre.