There are numerous synonyms for the word "self-activating" that can be used to describe something or someone that is able to function automatically without any external stimulation. These synonyms include "automatic," "self-starting," "self-sustaining," "self-regulating," "self-directing," and "self-operating." Each of these words conveys the idea of a system or mechanism that has the ability to manage itself independently. For instance, an automatic coffee maker is a self-activating device that starts brewing coffee as soon as it's turned on. Meanwhile, a self-regulating thermostat is able to adjust indoor temperatures based on the ambient conditions in a room. Ultimately, the synonym chosen depends on the context and intended meaning of the sentence.