I wonder if anything really can be simple. You can look at a blank piece of paper and think, “Well, that’s simple,” but in thinking that you’re ignoring the process that went into making that piece of paper. First it was a tree, then it was cut down, then it was taken to a paper mill where it was cut up into perfect little pieces of paper. Something similar could be said about a puppy, for example. Sure, on the surface it’s just a cute, innocent little animal. There’s nothing else to it. But you’re ignoring the fact that, as with all living creatures, a puppy has all kinds of body parts working together and within each body part are different cells, etc that keep the animal alive. Even when you get down to something like water, it’s complicated. Water is made up of hydrogen and oxygen working perfectly together to create the see-through substance that we all need to survive.
So what is simple? Perhaps facts are simple. For example, I know for a fact that if I don’t get enough sleep I will be tired. Of course I say that and then I think, “Well, there are certain times where I get hyper even if I haven’t had enough sleep.” I do think that fact’s like 2 + 2 = 4 are both simple and true, however. The annoying thing is that there is always the question of why? We don’t need to know the answer, but the question is almost always there. I suppose the question and the answer don’t necessarily change the fact that a fact is a fact and you really don’t need to go any further. The problem is that by themselves, facts are simple, but there are so many of them that you can’t possibly keep track of them all and therefore, they are quite complicated, especially when you try and combine them.
I tend to jump right to abstract things like art or music when trying to answer this question, but behind both are theories and techniques and even sciences that complicate them very much. Another answer that I’m sure many people would jump to is love. I suppose you could argue that it is simple in that sometimes it just happens and it works out, but this is the ideal situation, and it almost never works out perfectly.
Perhaps time is the answer. All it does is go at a constant rate. No one can alter it, and no one can make it stop. It has always been there, and it always will be. It is a very significant part of the world, and yet it doesn’t care what goes on in the world because it doesn’t know what goes on in the world. It’s just there. Evidently, not even God will mess with it (not directly anyway). You hear of prayers being answered, but you never hear about time stopping. Therefore, I’d argue that time is the simplest thing in the world.