I do a lot of retrieving with one of my dogs, and she gets obsessive very fast.
So we mix it up. She sits and waits while I chuck out two or three canvas dummies, and I then I direct her to fetch one of them, then another. I'll send her for one, stop her, redirect her. These are all what are called seen retrieves.
We also do 'blind retrieves'. I'll sit her on the edge of the woods, go into the trees, put out a few out of her sight, and send her out for them. Or I'll go over the lip of a hill out of her sight, and chuck a dummy over a fence or ditch, call her in, and send her for it.
Something else you can do is a memory retrieve, where you put out an item you want a dog to retrieve with the dog watching, then walk away with the dog and send it back at whatever distance works.
And as a PP suggested, you can hide items and get a dog to find them - it's wonderful to watch a dog hunt a patch of ground, suddenly smell what it's looking for and race in to grab it.
All of this provides a mental as well as a physical workout without putting massive stress on their joints. And it can sound amazingly tricky, but if you build up to it gradually with a dog who enjoys retrieving, it's eminently possible.
After a session of some of this, I have to chill mine out with sniffing for treats and some free running.