In regards to the idea of using old technology for currency, I think that those teeny little button batteries would, aesthetically, fit quite well in terms of being currency. And if you were to have greater forms of that same currency, you'd have bigger batteries (or different types entirely). I feel like you could probably incorporate a typical battery checker somewhere in there somehow, whether it be for measurement, or, well... I don't know.
Were you to use flowers, I'd imagine that the petals would be most effective, but entire flowers would work as well, I suppose. You'd have to have a nicely-reinforced patch to carry currency like that, designed so friction would tear the flowers apart....
This is a rather strange idea, and I'm not sure how you would go about executing it, but what if you were to use gigabytes/megabytes/etc, stored in some sort of memory stick or other piece of hardware (though this doesn't entirely make technological sense), as currency? Or maybe use files that would take up space in your character's storage device(s) as a sort of currency?
You COULD even use something intangible, such as "inspiration," as your currency. Gathering inspiration and giving it to painters or musicians, for instance.
Anyhow, that's my two pieces of undecided currency.