I kind of like the solution adopted by the Polish city of Wrocław (and many others, but Wrocław is the one I'm personally familiar with.) Their transit system

I kind of like the solution adopted by the Polish city of Wrocław (and many others, but Wrocław is the one I'm personally familiar with.) Their transit system relies on bog-standard bank-issued Visa / Mastercard payment cards, the kind most Polish people already have in their wallets. When you buy a ticket (usually at a ticket machine at the stop or in the vehicle itself), the machine temporarily remembers some of your card information. If a ticket inspector comes, you just tap your normal credit / debit card to their terminal, which queries the vehicle's systems somehow. It's basically the reverse of a storage-value system, they use a payment card for a kind of identification. All transactions are performed offline, usually the next day. Tickets are cheap enough that fraud is not a big concern, and there's a system in place to blacklist cards that can't be credited. If you end up on that blacklist, you have to go to their website or office, enter your card number / tap your card and pay the missing amount. If you're a resident and not a tourist, there's also a system of transit cards I believe.