Well, after bugging my teacher, I have finally been allowed to perform in the school talent show. My principal has to safety check my audition, which means no ledges, street, trials, riding skinnies or grinding 
What kind of really really easy flatland tricks could I learn in 5 days? Also, what sort of “routine” could I do? (usually I just go o the skatepark and do my own thing in no specific order…)
Do whatever you can do. You probably think most of what you can do looks boring because you can already do it, but to people watching you perform they will hopefully appreciate whatever you do for them. The more talent you have, the more talent you can show. Don’t be too scared to try something you can’t do well- if you can get it on the third go it will build up suspense and show people how hard unicycling can be. If you have never performed for a captive audience before it might be a strange feeling to have all eyes on you. Try to get used to the eye pressure if you can.
From your post I can’t tell what level your riding is at- maybe you could just do your own thing in no specific order as your performance- do what you do best.
Don’t bother trying to tell people you are not in the circus. You are on a unicycle and you are performing and it is up to the audience to judge what you are. The more you say you are not a clown the more you will get called it. Try to take it as a compliment even if someone says “circus” or “clown” in a negative way.
I can’t help much with flatland, but I’ve been performed in a talent show before. Any ‘skit’ that involves the audience tends to go pretty well. For example, you could start your act by having some volunteer hold your unicycle so you could run up and suicide mount it - that act could have some really intense build up if few people have seen you on a unicycle before. Jumping rope (two volunteers swinging - have the audience count) or jumping over someone lying down are also fairly simple to do and yet have great audience impact.
I suppose that it would be a bit different from the type of riding you do for fun, but my impression is that if you do a purely flatland act you could end up being on stage just just to show off and impress rather than entertain.
This article is about juggling in performances, but a lot of it does apply to unicycling as well (there’s a difference between doing either of these on stage for an audience as opposed to doing it for fun or competition):
http://www.nd.edu/~juggling/artperform.htm#showcontent
edit:
a quick search turns up this thread: America's Got Talent Looking for Unicyclists about Jason Auld’s Britain’s Got Talent performance. Mikefule posted something useful: