What's your biggest parenting challenge right now and how old is your LO?

Right now we are struggling with transitions - like getting T to stop doing an activity he wants to do. Like leaving the park, leaving to go to errands, going indoors after playing outside, getting out of the bath. I give him a heads up as to when we are going to stop the activity and what we are doing next, but if he is having fun, he throws a serious tantrum. He is 21 months.