I am a big fan of the Tumi Q tote as a travel handbag. I don't always travel with a handbag aside from my computer bag/boarding tote, but when I do it is ALWAYS one of these. It is roomy, folds up super well (fitting either in my main compartment or the front compartment of either my boarding tote or rollaboard) and works for both casual and dressy outfits.

I used to travel with a lululemon bag that folded up into an attached pouch but fell in love when I met the Q tote - now I own 3 in various colors.