This is a really interesting thread. Thank you for posting it.
In regard to songs to play...that is fun to read. If there was a definitive "single" song, every music store in the world would get tired of hearing it. Like most others, I would suggest playing through most of the chords you know (which means that you also play in the music that you normally sing), and to finger pick a little. The suggestions of trying a barre chord...I might also try "chunking" or "palm muting" an instrument to simply get an idea of how the instruments sounds.
Regarding demo videos, Barry Maz (gotaukulele) plays the same progressions and patterns on every ukulele (on those he likes more, sometimes he goes further). So far, regarding TheUkuleleSite/HMS, the only negative about their videos is that they don't do that for those instruments--they have members of their staff who are pro-level players demonstrate what the ukes can do (I would like that PLUS a clip showing a standard progression, which is how most players will play, that is played on every ukulele for comparison purposes). And that is the only negative thing that I will ever say about that company (and that isn't much of a negative).
As for wanting to impress the store employees--I "get" it. As to what others have said, you have to "get over" that feeling yourself. First, if you're someplace like TheUkuleleSite, most of those folks are pro-level players that gig at night or have made recordings. Even the guys that say they don't play, play. And it makes sense--they are in a store that generally deals with a single instrument.
If you're in a standard music store, there is a better chance that the staff doesn't play the ukulele and sees it as a freak (Tiny Tim) or folk instrument, and regardless of what you do, you can't impress. Music stores love ukulele because they sell a ton of them and it helps the bottom line.
All that said, looking down on someone due to their playing ability is an incredibly immature, self-focused, and uninformed thing to do. The better the player, the more they should be interested in helping propagate the instrument and helping others to get better. Like in all parts of life, there are jerks everywhere, particularly in music where it can be a fight to "make it" and to "survive." College music major are often pitted in a drama where they are fighting with each other for opportunities. Incidentally, all of this is in opposition to the general practice and performance of ukulele.
And a good music store should be concerned about helping you find an instrument that fits you...something you like to look at, something that you like to hold, something that sounds good to you, and something that is going to keep you coming back to playing. They should want to sell you a ukulele--but to sell you the right ukulele for you. And that means letting you play it before you buy it, and dealing with whatever you choose to play. And if you can't play yet, demonstrating some basic chords and giving you resources before you leave.
Incidentally, Kala's new free ukulele lessons are going live. I'm looking forward to going through them--not because I am likely to learn anything new for my own playing, but so I can see how they approach things to be able to help teach others.
Sorry--I'm a teacher. I'm really passionate about accepting everyone wherever they are at and helping them improve.
