In the interest of making a plan, and thus having a purpose to plan for, as suggested in the previous post, suppose we attempt to view the game world from its simplest perspective.
Imagine, if the gentle reader might, that you are seated upon a very large, endless floor, that goes off in every direction without boundaries. Imagine further that you are about 11 months old, on the cusp between infant and toddler, where you have managed to get up on your feet for a few steps, but that you crawl like the blazes. Finally, let us grant that you have a little cognitive ability, perhaps a bit more than you reckon an infant has; you can mumble out a few words, you can recognize familiar objects and you have a comprehension of time.
Now, you're not particularly sleepy, and every direction looks precisely the same, so what do you do? Well, off hand you have some options. You can first see how far your feet, hands and knees will take you. Given that using these seems only to present the same horizon as it did when you started, you soon tire of this except in that you're simply enjoying the process of movement. When that enjoyment runs out, you can sit again, that being comfortable. Now, next you can spend a little time playing with your fingers on the carpet, making patterns in the pile. After that, you're reduced to examining the various parts of your body - the ways you fold your legs and arms, the sounds you can make, waving parts of you about, and so on. Eventually, you'll settle down to a good game of "let's see what's in the diaper," which will take you a while.
In the end, however, if the circumstances don't change, you can holler for attention. This usually results in your immediate environment changing, and that is a good thing.
Look at that collection of options: travel, creativity, self-examination, investigation of mystery ... then, finally, a call for help.
Let's step out of the baby's mind, then, and put yourself as you are now in the picture - only, as it happens, you're utterly invisible to the baby. Of course, you wouldn't normally be, but for our purposes we don't want to get wrapped up in the baby's reaction to you. Let's say, instead, that you're simply here to place things into the baby's life that are going to matter to the baby. What can you put there?
First, most obviously, any object at all. That gives the baby something to walk or crawl towards. You could give it directly to the baby, but consider how much more involving it would be to place it just inside the baby's cognitive knowledge, maybe twenty feet away. Far enough away that the baby might start towards it, get tired, lose interest in the object, rediscover it, then start forward again ... only to happily seize the object once having covered the distance.
Now, what is this object? Is it static, unchanging, or is it like the carpet, with a pile that responds to the baby's actions? How effective is it in allowing the baby to experiment with it, changing how it is shaped, how well it bounces when thrown, what sound does it make when squeezed and so on. These are all important elements in how a toy responds when a baby interacts with it ... for good reason. The sounds interest the baby. The create memory patterns, so that when the baby sees the object again, the baby remembers not just that it is interesting, but what can be done with it. Moreover, if it allows lots of experimentation, or is something that takes great skill to manipulate in every way possible, the baby will keep coming back to it for a long time.
We might put out a bunch of objects, to allow the baby to 'choose' depending on the object's color or shape or versatility. We might stagger the objects in lines, or in circles, so that as the baby moves further out from its starting point, it sees different things. Perhaps the objects are more interesting and colorful in the outside circles, and less so nearer the baby. Perhaps you might try the reverse. What will give the baby the best overall experience?
Let's move on to self-examination. Let's suppose that as the baby moves something, you laugh and the baby notices. Or let's suppose that when the baby touches its hands together, you tickle the baby. Suppose that for each different behavior, you offer some comparison behavior that amuses or rewards the baby, and that on the whole the baby is encouraged to touch its nose or rock on its bottom.
Now let's say that when the baby reaches its hands into its diaper you slap the baby's wrist. Or say a mean word. What is the effect of that? What is the effect of compelling the baby to sit in one place until you're ready to let the baby move? What happens when you take a toy out of the baby's hands? The baby doesn't understand why you're doing these things, so if they don't seem to happen for any reason the baby is only going to get unhappy. The more random you are with your negative actions towards the baby, the less the baby will understand and the more frightened or angry it will become.
Now ask yourself. What is your world meant to do? What is your world meant to offer? How logical or rational are the rules about what people in your world can't do? How randomly do you apply those rules? How long do your players have to wait before something happens? How much is there for them to do? How many choices have you offered? How interesting are the choices? And do the players understand why you let some things happen and not others?
Finally, since your players aren't babies ... how often do you give answers to the above questions while forgetting that most salient fact?