Hi Kelly:
You can base the transition from MCL (Marketing Captured Lead) to MEL (Marketing Engaged Lead) based on any activities that are relevant in your buying process. Particularly when it comes to events, you can leverage the Program Statuses; "Visited Booth" at a trade show si certainly one of the more common ones, downloads .PDF is another common option.
Keep in mind that most of the valuable activities use registration forms in one way or the other, so the Fills Out Form trigger is fairly universal.
Hope this helps.