DH grew up with the rule that you can't open the toy until you've eaten.
I didn't get Happy Meals all that often growing up, but I know our family had the rule of no books/toys at mealtime.
We haven't been consistent, but I think we're trying for "no toy until you're done eating." (This does not mean the food is all gone, just that the child has eaten what they wanted.) Again from the time perspective.
Since DS is under 3, we also run into the issue of MANY, MANY, MANY fast food restaurants ADVERTISE to "ask for special toy for child under 3," and then when we ask, they say "We don't have any."
The last "toy", though, was a CD of audiobooks, so not much for DS to play with there.