Drat... Why do you have to be knowledgeable about the piano! Grr...
So that leaves the organ... Ever seen a pedal glissando? People are amazed but they are even easier than glissandos on the piano. Seriously, they're simply awed and I think this is the easiest pedal part in the entire piece. Hahaha...
Pedal trills amaze people too, but they're really actually very easy. All you have to do is get the coordination, and they are almost easier than trills with your hands.
Nobody ever appreciates just how difficult parallel 6ths are on the organ... They are probably the hardest thing you ever have to do on the manuals.
Can I talk about piano now?
Okay, I think arpeggios are the "showiest" thing you can play, but they aren't even that hard... =] Another thing is tremolos. People always say "You were rockin' with those shaking things!" Of course tremolos are perhaps the most difficult thing for me, but people are amazed at them.
People think trills are just so spiffy, and I think they're the easiest thing in the world.
Now trills in 3rd or sixths. Yikes! Those are insanely difficult! And people don't seem to notice them as much as normal trills... Too bad.
Octaves sure are impressive. I've played the Liszt 1st concerto for some folks, and the octave deals where your hands fly back and forth... They're like "Whoa! You're hands are moving so fast!" I say, this really isn't too difficult...
Now... You know the Liszt transcendental etude 5? Doesn't seem to be very hard and, well, IT IS! It's not a good show piece because it harder than anything else in the world and it doesn't even sound that impressive...
http://www.youtube.com/watch?v=iZj-8x5i6Bs
There are a lot of pieces out there that people think are easy but they are really, really, REALLY hard! I'm thinking at the moment of the Schumann piano concerto. I've played a lot of concertos. The Liszt as I said before, Grieg's, Beethoven's 3rd and 4th, Chopin's 1st, Prokofieff's 1st, Rachmaninoff's 1st, Mendelshonn's 1st and some others, and the Schumann is one of the hardest of all of em'! The difficulties are hidden. It is so hard to put the passion in the notes... So hard. While it's not especially technically difficult, I think it is the most musically demanding concerto of all of the above listed.
Interesting question!
--Schumiszt