The Sonoff Mini is, as the name implies, a very small device, designed to be hidden where there's only supposed to be cables. Since it's not supposed to be visible, it's also designed to easily attach an external button.
Tasmota can be flashed on this device via OTA using the Sonoff DIY feature.
Several free GPIO are available. The OTA jumper pin is the most accessible and very easy to use on a pin header.
|4||S2 (external switch input)|
|12||Relay and red LED|
|16||OTA jumper pin|
|GND||S1 (external switch input)|