We haven't done this quite yet, at least not "formally", but I'm thinking of taking retrieving up a notch. Or at least take it along a different path of sorts. We've worked on directions and used retrieving to help teach him that (something ELSE we need to get back to), but I've also thought about having him retrieve multiple objects.
He wouldn't try to get all the objects at once, but get one, come back, go get the next one, come back, etc. This would be a different spin on it and something that would challenge him in another way. He would have to remember where the objects landed (and so will I for that matter, and now that the grass is getting longer in the fields and such...) and go and get them as I indicate the direction to start in.
This sounds like it's going to be quite the challenge for the both of us, so starting off slowly is a certainty. Slowly as in two objects only and in an area with the shortest grass possible!
Then as something of a bonus, if he gets mixed up, we can work in directions as I try to guide him to it (which is going to be an adventure since we haven't exactly mastered that too much, but at least he's not completely clueless of the concept). Always nice when one activity can focus on one skill and then mix in another one as well.
I can also imagine throwing in that color discrimination here too. Have go find the blue or yellow card or object (can't wait until he gets good at the colors - lots of ideas).
But first, I have to get him into going after multiple objects. Shouldn't be too hard, but you just never know. Some things I've thought would be easy for him weren't and some things I thought would stump him, he picks up fast. Just never know.