beside the whole guild bonuses and personal rewards -
i think adding in shop option for leader to buy something to reward member is another good idea
(or for that matter option for a member to buy something for another member, restricting it to leader alone would be a stretch)
about guild activities as a whole, for now i am looking at the guild wars…
but yeah beside that maybe a pve so guild raids? 
some day
also i think very cool would be a version of “console tasks” given to guild as a whole!
in fact ill try later today put all of the stuff i mentioned here in one feature request thread