Activities are what the person, system, process is doing at the instant of observation. We want to keep the activity list as short as possible since the more activities the more observations needed to get the same accuracy.
One way is to start with an empty list and write the activities down as observed. Another way is to start with one of the templates below (already loaded by ISampler on your iPhone). Edit the list until it is as short as possible but covers the activities that you want to study.
A great idea is that you can increase the detail when you are studing specific areas (such as a study of just waiting time where you list activities like waiting for a crane, waiting for an electrician, etc.) but keep it more general for general studies.