Follow these steps to add activities on the library.book.rent model:

1. Inherit mail.activity.mixin in the Python definition of the library.book.rent model:

class LibraryBookRent(models.Model):
    _name = 'library.book.rent'
    _inherit = ['mail.thread', 'mail.activity.mixin']
...

2. Add mail_activity widgets in the chatter of the library.book.rent model:

...
<div class="oe_chatter">
    <field name="message_follower_ids" widget="mail_followers"/>
    <field name="activity_ids" widget="mail_activity"/>
    <field name="message_ids" widget="mail_thread"/>
</div>
...

Update the my_library module to apply the changes. This will display chatter activities:

This is how the user will be able to manage different chatter activities. Note that an activity scheduled by one user is visible to all other users too.