Weehiggers wet bag makes a perfect potty training bag!
My 27 month old is out of diapers but has a hard time in public bathrooms. I bought a folding potty seat for her but didn't have anywhere to put it. I brought out the wee hungers wet bag, but the potty seat, wet ones and toddler wipes in the wetbag part and a change of clothes, undies and a small wetbag in the front pocket. I, also included a small thing of post-it notes for automatic flushing toilet situations. (put it over the sensor so it doesn't flush while your kid is sitting there).
I keep it in the car and bring it with me if I think we'll need it.