Add solar panel system to shed to provide power down at the patio.
Would need a solar panel (50W?), charge controller, battery (deep cycle lead acid) and an 240V inverter to run mains powered devices.
Could get a submersible pump (are 12V ones available?) to put in the water barrels to use to water the veg patches better. Could this be automated with Raspi/arduino, solenoids and soil moisture sensors to provide automatic watering? Could tie it in to weather forecasts?
Get EL-wire and fix to outline of shed and power from the battery to provide funky lighting at the patio during the evening.