The trainer who shared this advice with me had been owning working dogs (Malinois) for many years. She knew them inside and out..
And her advice was spot-on. Do you have a really intense, high-energy dog?
Running around and jumping and being wild is easy for them. Fetching a ball for 30 minutes is easy. Rough-housing with another dog for an hour is easy.
What truly challenges them is what they can do the least well: Being still and observing. And the most challenging tasks are the ones that are the most tiring (for both humans and dogs).
If you have never done this before, give it a go!