Gadgetbridge shows heart-rate and activity when not wearing the watch

I think the activity is usually calculated from the movement of the watch - not steps per se.

Is there a temperature sensor that reasonable reacts to skin-contact and could provide some context?

There is a sensor in the watch, but it's deep inside so would only react very slowly. Using movement is probably the best bet - an easy proxy is whether the 'poll handler' is in the high speed (80ms) or low speed (800ms) modes