I’m a huge fan of the Pasmo/Suica cards. They’re easy to obtain and easy to accept payments, so uptake is huge (if memory serves me correctly, literally accepted everywhere?). You get most of the benefits of card payments (fast, no fiddling with change, no “do you have a smaller note”), but you also maintain anonymity which is a common argument I hear against cards in Germany and why card payments are so rare there (at least in 2017, I don’t know if the situation has changed).