Goal: use a trigger to update a custom field on the Task (OppNumber) with a field on the Opportunity that is selected in the WhatID field (if an Opp is selected)

I thought this would be easy to do using a lookup and just passing the value, but you apparently cannot create a lookup on the Activity/Task object. 

I know I can pull in the record ID for the opp related via the WhatID field, but how can I refer to a specific field on that record? (Such as the Opportunity Number).

Open to suggestions.