e-Paper displays have great readability under sunlight, and only consume power when updated. But their refresh rate is limited, and most displays are fairly expensive. One way to reduce the cost is to completely remove the battery from the display, and update it through NFC providing both data and power.
I've just found out about that possibility through 4.2" and 7.5" displays offered by Waveshare. All you need is a compatible NFC smartphone, or the company's ST25R3911B NFC Board to power the display and transfer your image of choice. This works best for signs you may need to infrequently