A small electronic device (around the size of a microchip card) that you can top up with a particular currency by simply touching with your contactless bank card or a smartphone through a banking app. You spend the money available on it the same way - touch it to any chip and pin reader and pay for a product/service.
The device has no Bluetooth, wifi, or remote connection of any sort apart from contactless loading and discharging of monetary value. It is not associated with your identity and keeps no record of transactions, only shows the amount of currency it currently contains.
Replace cash with something material and conceptually and functionally similar.
No need for bank accounts, electronic wallets, etc. to use the device and pay/get paid.
Maintain anonymity when paying/getting paid.
It is a solution to this problem.
To enable top up with contactless bank cards, smartphones, smartwatches, etc. the device should work by a similar principle that chip and pin readers work, so it's pretty clear that a simple microchip (like on a credit card) won't do. It has to have the means to set the amount that will be loaded on it through contactless "touch". It also has to show you the amount currently loaded on it.
I think a card as thick as 2-4 standard microchip cards would do to hold the required electronics in place. You could go about different ways on how to set up the user interface, but I think the best would be to implement a small touch screen.
You'd activate the touch screen by entering your pin code. The screen shows you the amount of currency on the device. There is an "accept top-up" button. After pressing it a number keyboard shows up and you enter the exact amount to be loaded on the device, then press "confirm".
After that, the device is ready to accept contactless payment/top-up through various means. You can cancel this active mode by pressing another button. The device also cancels it automatically if it doesn't receive contactless transaction in due time.
Another interface option is to use a non-digital screen and a few buttons (a tiny physical keyboard). This might let the battery hold longer. The battery could be rechargeable or replaceable. If it was disposable then the device should warn the user well in advance to take the money out of it before the battery dies.
When you want to pay using the device, you don't have to do anything apart from touching it to the chip and pin reader (just like contactless bank card).
There could be separate devices for separate currencies (EUR, USD, Bitcoin, etc.) and I think it might be a good idea to set the limit amount that can be loaded on the device. That way it would be less of a detriment to lose the device. Similarly as banknotes have different monetary values, different devices could have different limit amounts to load on them. You'd use some for gathering and paying smaller sums and others for bigger transactions.