The easiest way would be to copy the keystone theme (I assume you are using that?) and then add {activity_link} right below {discussions_link} in "themes/your-new-theme/views/default.master.tpl"
If you want to add kinks to other/custom stuff(f.ex a rules thread/page, about section or external links) this might be the thing https://open.vanillaforums.com/addon/index-plugin