You toss a coin in a perfectly isolated box...you want to check what face it shows...
With the logical underpinning of this question, you have already come to a problem. Who is to say the coin even shows a face? Who is to say the coin ever landed, as there is no air in the box to carry the sound of the coin landing to you? Perhaps the coin is still falling through the air; maybe its floating close to the ceiling, or maybe right above your hand, or it disappeared altogether halfway through its trajectory and was replaced by a "very surprised looking sperm whale".
"AHA," you say, "but I know the coin MUST have landed, because the box is on earth, and on earth, there is gravity, which would have brought it down! And I know it must've landed on one side, or the other, or maybe even on an edge (although that would be quite improbable) simply because there is no other option for its landing!"
But in that case, your isolated box was not so isolated after all, since inside of it, we could still feel the effects of gravity...(more on this later)
What determines isolation for you? No air, no light, no sound? No interaction of any kind?
But yet you claim the coin has landed on one side or the other, which requires some sort of interaction between the floor of your box, and your coin.
If the coin indeed HAS landed, then it does not require that a conscious being check which face of the coin is pointing up (whatever UP can even be in a "perfectly isolated box"), or check where on the ground the coin has landed, as the ground itself has already checked.
The electric subatomic interactions that keep the coin from falling through the ground of the box itself have been established, and had the coin fallen on its other face, or on its edge, or in a different location altogether, those interactions would have been established differently, with the pressure distributed in a different way, and the atoms at the location of the collision would have vibrated in a different way, and so on and so on:
As long as there was any sort of interaction and a way to differentiate between one event and another (the coin landing on one face being one event and landing on the other being the other), one of them MUST'VE happened, and regardless of whether or not a conscious being checks, one of the definite events will have taken place.
It's a really good question, the one you are asking, and one that in my opinion has created a lot of confusion: I will change the scenario for you a bit so that the problem becomes more apparent.
Instead of a coin, you throw a chihuahua (bear with me) up in your ideal box, and you don't hear it land, since there is no air for the sound of it landing to propagate, yet you claim it has landed and want to know whether the chihuahua has landed facing up or facing down.
The situation hasn't changed much from the tossing of a coin, but now you know that the chihuahua could not really be in a superposition of up or down at the same time, since he/she (emphasizing consciousness) must be conscious of their landing position.
Does the fact that the dog was conscious, but the coin wasn't, really change the situation so much that it now is objectively determined? Had the coin been conscious, would it be meaningful to postulate what side it shows?
No, the coin HAS landed, and it has picked a side, but it's not because your theory is necessarilly wrong (after all, Quantum mechanics IS a thing), but because you have the wrong interpretation of this uncertainty (although many physicists and philosophers alike would still disagree with me).
As long as the event has happened, it has happened, that isn't where the uncertainty is. You can only live in a world where one event or the other has taken place, it's not checking whether or not the event took place, that makes it take place.
But the reason for this is because there is no way to isolate your box of all interactions: regardless of what you do, the coin will have interacted with the floor, or with the ceiling, and it must've landed in one position or the other once it has landed, and affected the world around it in different ways.
Now, if there really were no interactions in your box, there would have been no way for the coin to land in the first place, meaning the event has not taken place. Here is where the uncertainty actually does arise: if we turn on interactions, so that the coin can indeed land, and it has an equal probability that it lands on one side or the other, which side will it land on?
There is no way of telling, and here is where the awesome theories come into play: like that two parallel worlds will be created, one where the coin lands on one side and one where it lands on the other, and you will only exist in one of them, with an identical version of you in the other where the coin landed on the other side.
But at the end of the day, you will only exist in ONE OF THEM.
Hope that helped!