A more lively and interesting song than most that would be played there.
Be Quiet and Drive (Far Away) (Acoustic)
An awesome song, it just depends on the crowd weather to play it or not.
Good song, boring choice. See also Hey Hey http://tinysong.com/2JSC
Handbook for the Sellout
I think you could totally translate the horns into a second guitar.
For silliness, see also Oh Canada
They've got trees and mooses and sled dogs, lots of lumber and lumberjacks, and logs! We all think that it's kind of a drag that you have to go there to get milk in a bag.
Death from Below
It's hard to find anything by these guys. This is one of my favorite songs of theirs, and would be only moderately challenging to convert and play.
Since I've been somewhat fixated on the Decemberists lately, a few other songs of theirs that would work well:
Los Angeles I'm Yours http://tinysong.com/1YBK
Shanty for Arethusa http://tinysong.com/1qus -it would be shorter and need an accordion, but it would work
Billy Liar http://tinysong.com/29Ky
Red Right Ankle http://tinysong.com/2zyZ -way too obvious for a coffee shop
Summersong http://tinysong.com/1qoq -not necessary, but accordion would be good on this one too
Yankee Bayonet (I Will Be Home Then) http://tinysong.com/2vsV -also pretty obvious
Sons & Daughters http://tinysong.com/2JTM -only if you've got a chorus with you