Personally, I think the braille style bumps idea is be best, but they could wear off, or be compressed down with age. The foil idea makes me thing the bills would wear out quickly. That leaves differing the sizes of the bills, like the rest of the world does. I think that might be a hard sell, though, given all the vending machines, ATMs, money counting machines, basically anything mechanical that handles mony will need to be replaced nation wide.
How would you make dollar bills usable by the blind?