I was thinking the antigua would be a great gym bag. You can probably wash it as well more easily than the damier or monogram. Or for the ULTIMATE gym bag, why not use a Monogram Marie or any of the Vernis Tompkins Squares? They already have the shape!
I think the best throwaround bag though would be a speedy 35 for the gym.
I think the best throwaround bag though would be a speedy 35 for the gym.