You'll see lots of cool stuff... but you probably won't get to do much of anything.
That is to say, you'll fetch cups of water and boxed lunches for patients. You'll deliver X-rays taken at other hospitals to radiology. You might be asked to wheel patients through the hospital if the hospital doesn't have an official Patient Transport service. And you may babysit children while their parents deal with health issues or go out to talk on the telephone. None of that stuff is really very interesting, so don't get too excited about what you'll do.
What you'll see on the other hand can be pretty awesome. You'll learn about your hospital's trauma Red Phone and watch what happens when a call comes in with a trauma case: hordes of people gowning up, surgeons stampeding down from the 3rd floor, nurses & techs setting up equipment and getting everyone out of the way-- including you (You won't get to see too much that happens during the trauma, itself, though-- there's a painted line you'll have to stand behind that keeps you away from the action). You'll see drunk patients, high patients, neglectful parents, suicidal kids, the mentally ill, broken limbs, nasty accidents (I saw one kid whse flish had been ripped from the bone of his finger and it was all just hanging there like the skin ripped from a chicken leg), people with chest pain, and people with acute issues like appendicitis. Lots of fevers. Lots and lots of old people (many with various levels of dementia) who've been bouncing back and forth from the hospital to their nursing homes over the past 10 years.
You'll also see BS patients like hypochodriacs, people who simply have colds but don't have insurance to go to a regular doc, people who claim to be suicidal but really want "3 hots and a cot," and people who just don't have a clue what "emergency" means (Doc, I have this really bad sunburn!).
To learn from what you see, try to make friends with the 1st & 2nd year residents. This works better than honing in on the attendings, who are generally very nice, but are so surrounded by residents that they don't have time for you. If you're smooth, you'll manage to effectively "shadow" the younger residents, so you can watch them present their cases to the attendings. This is how you'll learn all sorts of good stuff.
As you can probably tell, I loved my time in the ED. Just don't expect to learn by doing, or you'll be disappointed. You can, however, learn quite a lot by watching...
Have fun! 👍