(Missable) The pink balloons are scattered all over Industrial District and the Shipyard. The locations are below in the video. You’ll need to use Robin’s shurikens or Batman’s batarangs (select either with on the d-pad or quickfire with ) to pop them. There are 30 balloons in all. Fortunately, if you don’t get all of the balloons in your first playthrough, you can start another one and your count will remain the same. All of the ones that you’ve popped will be gone. You’ll just need to find the remaining balloons.
Video (Thank you to 7he1With7h3F0RC3 on Youtube for the video):