The hide/show completed things is in the my settings area.
And it's important to know that the my settings show/hide is a global setting - so you want to predominantly show Or hide completed things.
When you need to see or not see things on an ad hoc basis, then you'll use the filters on the activity ctr to show or hide them -
as an example - you have chosen to hide completed things in my settings but occasionally you want to see them - you'll set a filter
You can save and re-use filters so you don't have to re-create them each time - see this help page for the steps on how to save filters