There are also people who go to farmer's markets and sell poems written on the spot on typewriters - http://www.flickr.com/search/?q=poem%20store - similarly whimsical, handmade, and suitable for impulse purchases for people out on dates or whatever. You could multivariate-test the sign.
Also check out the licensing requirements for street vendors if you haven't yet: http://www.nyc.gov/html/dca/html/licenses/094.shtml (to help make sure you know what you're getting into, and also because it has exceptions for artwork and written matter).
Case in point, Joe Ades with his vegetable peelers: http://www.youtube.com/watch?v=HCUct4NlxE0
I'm not sure if you've heard of Joe Ades. He used to sell potato peelers in NY and has been mentioned on HN a few times:
http://en.wikipedia.org/wiki/Joe_Ades
http://www.nytimes.com/2009/02/03/nyregion/03ades.html
http://www.youtube.com/watch?v=HCUct4NlxE0
I don't think it really matters what you sell, especially in this case where you just want to improve your sales technique. The main thing is that you sell an experience, not a product.
I can't find the exact news article, but this one is close ($200 profit/half day!):
http://www.dnainfo.com/20110628/midtown/kids-set-up-lemonade...
If you think about it then there are tons of amazing geometrical shapes out there that would make amazing trinkets, and why limit yourself to pure mathematics? Imagine creating a trinket that represents plasma captured in a magnetic bottle. That would be beautiful if you can get it just right.
This will be challenging, artistically, to pull it off (perhaps you can use a 3D printer), but if you do I want one. (yes, I have weakness for such trinkets)
https://beerfarmer.3dcartstores.com/Adventure-Socks-_p_13.ht... contact me through the site if your interested.
They hand you a CD and tell you they're trying to get their music out there, etc. Then ask for donations. I felt like a sucker but the lesson was worth it I suppose.
Lots more details:
http://www.huffingtonpost.com/2009/12/11/raymond-martinez-ti...
@murtza They do things like this in the Apprentice all the time. There's probably some good ideas there.