"Fetch the Brain" is something I do when at the park throwing the ball for the dogs. They say working a dogs brain tires them out faster than just physical exercise.
I'll throw the ball a few times for the dogs and then I'll randomly change it up by asking my dog to do one, a few, or all of the following... sit, down, spin, and back up. I will add in a stay at the end and throw the ball and have my dog stay until I release them to get the ball.
Another great thing to add is throwing the ball for your dog, having them go get it and having a command for them to stop chasing the ball a quarter, half way or three quarters of the way to the ball. I think is really valuable because it could come in handy later on. Depending on where you live (in the city) or even if your dog is big into chase, teaching them this can help if your dog ever sees something appealing and wants to dart out near traffic or chase something cliff side of a trail. Knowing Stay and Stop commands could end up helping you help your dog from a dangerous situation.
When teaching these kind of things, make sure you teach it in small steps. You don't want to throw a ball across a field for your ball crazed dog and expected them to stay, start with asking your dog to stay and tossing the ball only a foot away, close enough for you to be able to interject if your dog breaks from it's stay and tries to grab the ball. Over time you'll increase the distance you can throw the ball.
With training your dog to stop while mid chase for a ball, start with asking the dog to stop when the dog is closer to you than the ball. From there you'll build up to asking your dog to stop closer and closer to the ball and further and further away from you.