Cryptocurrency is, with few exceptions, looked at with scorn by most solarpunks. This is due to the technology itself being almost exclusively used by grifters and scammers, as well as some inherent problems to the technology itself, which are wonderfully explored in this Folding Ideas video on Crypto, DAOs, and NFTs.
But to answer your question, I think one of the few viable options would be something based on GNU Taler, which is a sort of anonymous digital cash that is distinctly not based on cryptography at all, and thus avoids the unbelievable waste of energy that most Cryptos are plagued with.