In the middle panel, in the Activity tab, click the Filter activity dropdown menu.
To include an activity type on the timeline, select the corresponding checkbox. Select the Select all checkbox to select all activity types.
Please note: filtered activities are set on an individual user basis, and will apply to all records for that object, but not to other objects. For example, filtering activities on a deal record timeline will apply the same filters to all other deal records, but will not affect contact, company, or ticket records.
For faster filtering, click the Notes, Emails, Calls, and Tasks tabs in the middle panel to see only the respective activity types.