The greenhouse has an existing temperature sensor setup consisting of DS18B20 1-Wire temperature sensors. They are fitted inside RJ11 female-to-female extenders, which is convenient since arbitrarily many sensors can be added by connecting many of them together via RJ11 cables.  There should be enough of those sensors to build an heatmap of the greenhouse for example. Those sensors could be read by bit-banging on one of the two fast GPIOs on the Galileo, or maybe a proper USB-to-1Wire converter is actually required for those long cables.