I can't figure out how to create a simple checkbox that users can check and
uncheck when needed.
First go to Office button > Word Options > Popular and check the option to
display the Developer tab on the ribbon.
In the Developer tab, within the Controls group, click the Legacy Tools button
(fourth from the left in the second row).
There is a checkbox for "Legacy Forms" in the top row of the gallery; to make
that operate, you must click the Protect Document button (further to the right
on the Developer ribbon) and protect the document for "filling in forms".
There is also an ActiveX checkbox in the second part of the gallery. That one is
treated like a macro by the macro security function, meaning that if the user
has macros blocked (which they are by default), the checkbox won't work either.
There's yet another scheme (which also depends on having macros enabled) at
http://gregmaxey.mvps.org/Add_Toggle_Objects.htm.