I'll second the Naots. They are a contoured footbed, like the Birks, but have cushioning, which I do find more comfortable. There are a bunch of styles. I personally find them a bit more narrow than the Birks, which fits my foot better. Dansko makes sandals, but I don't think of them as flip flop alternatives. Can you wear a negative heel? Earth shoes are still around, and i have found that i can wear them pretty comfortably, although it takes a couple of days to work up to having them on for a long stretch.