I haven't been to IOA yet, will go for the first time in a few weeks. But our town used to have an annual event called "Festival 9 3/4" where our downtown was converted to Diagon Alley and Hogwarts for an evening. I imagine the Universal attraction goes even more in-depth into making everything authentic - they have probably 100+x the budget and they don't have to tear it down the next morning.
The event was enjoyable to those who had no HP background, but parts of it were confusing for them. For example, I taught Transfiguration class, and a few of my "students" had trouble understanding what I meant when I asked which house they were from, or why I took points from their house when they were late for class. They were even more confused by Potions class, the actor playing Professor Snape was so in-character that he just criticized them even more when they asked questions.
A few people were also confused about what types of food to order at the food tents, but I think most people had fun anyway even if they did not know much about Harry Potter.
So if you have time, it might be worth it to watch at least one of the movies ahead of time. When I read the books, I started with book 3 (and that was when there were only 3 books!) and I was able to absorb and enjoy most of it without getting lost. I read it again after I went back and read the first two, and I had a much better understanding and appreciation of the details the second time. So it's not critical to have the background beforehand, but you will probably appreciate and enjoy it more if you do.
If you only want to watch one movie, I'd pick Prisoner of Azkaban (3) or Goblet of Fire (4). 3 has a lot about Hogsmeade, Sirius Black, and Harry's past; 4 has a lot about Quidditch, and with the Triwizard Tournament has a lot about different types of spells, magical tools, dragons, and wizards from other schools. They are also a little more "fun" to watch than some of the later films, the series seems to get much darker starting with Order of the Phoenix (5). Movies 7 and 8 (Deathly Hallows) take place largely outside of Hogwarts and don't really present very much background information on the wizarding world, so if you don't want to watch them all I'd skip those.