This is not the most important thing, just a matter of polish.
On the activities page, each column has a pulldown box on the right side of each column heading.
(1) problem #1: the first level of this menu contains only a single option - "columns". Why have this extra level of indirection?
(2) all columns present the exact same functionality. as a general UI rule, a pulldown menu should somehow be specific to the context where the pulldown is located.
It think the selection of which columns are displayed should be from a pulldown located on the toolbar above the activity list, as a peer of the current controls (quick edit, more actions, +manual activity). This is more discoverable, more intuitive, less redundant, and also lets the column headings be truly centered (currently the are shifted left somewhat to account for the usually invisible pulldown handle)
