If you're just talking about a field on the lead record that you can add notes to, and have them sync, then you'd just need to create a text field on the Lead/Contact entity in Dynamics and sync it with Marketo. Then any update to the field on either side should sync back and forth.
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As John points out above, if "Activity Notes" is a text field in Dynamics then it simply needs to be added to the fields you sync with Marketo from Dynamics.
If you are looking to highlight milestone activities taken by a Lead then I would recommend using Interesting Moments to bring important historical milestones right onto the Lead and Contact entity forms in Dynamics.
However, if you mean that you would like to add an associated Activity record to the lead or contact (kind of like the Marketo / salesforce flow step of adding a task assigned to a rep) then you would need to use a combination of a custom fields created on both entities (and sync'd with Marketo) as well as a workflow to create the related activity record.