I think a lot of what one would want in it, would be very personal. I see people who come with most everything and those who travel light! Also, what you need varies, depending on the class and the project.
I'm thinking of making a bag and have been thinking a lot about what needs to go. If I make one, I want it to have EVERYTHING that I need to take in, except for the sewing machine. I hate having to make several trips back and forth to the truck!
Some things that come to mind (some already mentioned, I know)
* sewing machine manual, attachments, foot pedal, power cord
* large and small cutting mat, rulers of assorted sizes
* notebook, pattern for project
* fabric and then the finished/WIP to take home (perhaps a plastic container to put it in, and then slide into the travelling bag)
* rotary cutter(s), scissors, snippers
* different types of pins
* tea mug, mug rug, tea bags and lunch bag!
* bandaids
* tool box (I have a small divided tool box (about 8"x8"x2" with divisions with most of the miscellaneous stuff in that I need ... and try not to take from it when I'm at home)
* hand lotion
* ca$h and/or credit/card! (I never take in my purse, and download to a smaller mini bag)
* car keys (snap hook to clip them to, so I don't lose them!)
I know there's a lot more!!