Haven't tried this myself, but would giving the role permission to all of the major categories and the deselecting the sub-sections work?
So for example if you:
1) Check the box for
Marketing Activities2) Leave all the sub-sections blank.
I would assume that would allow them to view various items in marketing activities without the ability to make changes.
You could always test we a 'fake' user and see what you can/can't do.