If i "hide this stream", how can i make it visible again?
For now, there is no way to unhide invisible activities. You need access to database to achieve it.
Go to table #__community_activities_hide then find your user_id and activity_id and remove matched row.
i see no way to delete entrys or remove row...
You can use this query, paste it to query box and change table name, activity_id and user_id as you need
DELETE FROM `g7r1q_community_activities_hide` WHERE activity_id = 1 AND user_id = 2