You can search the activity stream by keyword(s)

You can filter by activity type. For example, if you only want to see notes, you can easily apply this filter.

You can also filter your activity stream by the user who created the activities.

If you have a lot of users, you will need to scroll down to "Add All" or "Remove All" and then search users to apply the users you want to see activities for rather than clicking the "x" for each user.

You will want to Save the filter. This filter will be maintained when you leave the page and return. It will also be applied when you are on another page that supports the filter options (i.e. contact records, opportunity records, project records and the Home page)