Mount an electric bike on an exercise stand, tap into the 3-phase motor with a DC rectifier ($10 on Amazon), wire up some voltage regulators, and pedal. Electricity will come out at the other end.
100W for one hour is a decent workout for me. 10 hours @100W average output yields a single killowatt-hour. The first generated killwatt-hour ran a 10w fan while working out, accounted for losses, and kept my iPhone SE running for two months under normal use.