@irontoenail - A no frills version of that is to play "memory" with kids, where you show them a tray of random objects for a minute and then get them to repeat what objects are there.
Reading is also very good for mental endurance, for children and adults. If you find your attention span is growing shorter, I would find the most interesting book possible and get yourself to read it. Or even start with short stories.
And don't think that video games or movies are a substitute for them. There is very little need for mental endurance when consuming media like that because you don't really have to concentrate at all. I'm very fond of movies and computer games, but if you want to train your mind to stick to a task for a long period, they aren't the way to go. If anything most of them will erode your ability to concentrate.