Connecting a simple device with my secured home wifi
If the device is sophisticated enough to connect to Wi-Fi, it probably has some fairly powerful microprocessor to implement enough of the stack to be able to do this. If that is the case, maybe you can support an AP mode in the device (it acts as an access point). In this mode, it exposes some trivial listener on 192.168.0.1:80 which serves up and processes an HTML form. Voila, you now have a keyboard and screen with which you can configure client credentials. A sliding switch on the device can toggle it beween AP and wireless client mode.