I like the shopping bag idea. I use shopping bags all the time for groceries and subsititute tote.... so that'd be a great way to give something useful and "be green". Great idea. And packed with a mini survival kit is just awesome & thoughtful.
A convention pin would also be great. Something nice, classy, & elegant that someone can wear to both represent the org and remember the event.
A nice keychain... I technically would consider mine to be a piece of crap plastic keychain, but I love it because when my heavy keys sink to the bottom of my purse, the plastic floats up and I can find my keys. It's awesome. Completely beat up, but awesome.
I think shirts are hard because it seems like there are only extra small and extra larges left at the end of the day. Ignoring that challenge, a hoodie would be awesome. I would also use a hat, but I know not everyone's a hat person. Here in Seattle you never know if it's going to be rainy or sunny throughout the day, and a hat protects me (and my glasses) from both!
Oh- one of the few formal favors I still use is a small jar like
this. We store our dogs' training treats in there to keep them fresh & accessible
They're relatively cheap, too.