To mark activities as done using Python in Odoo, you can use the write method on the 'mail.activity' model to update the activity's state to 'done'. Here is an example:

 Import the mail.activity model
from odoo import models

# Get a reference to the mail.activity model
activity_model = self.env['mail.activity']

# Use the search method to find the activities that need to be marked as done
activities = activity_model.search([('state', '=', 'to_do')])

# Update the state of each activity to 'done'
activities.write({'state': 'done'})

Alternatively, you can use automated actions to mark activities as done. To do this, you can create a new automated action with the following properties:

• Action Type: Update a record
• Model: mail.activity
• Record: Current record
• Set the following fields: state: Done

Once you have created the automated action, you can then use it in your workflow by adding a new transition with the following properties:

• Trigger: On Change
• Filter: approval_status = Approve
• Action: The automated action you created earlier.

This will mark any activities associated with the record as done when the approval status is set to 'Approve'.