What's an easy way to export all my data in MARKETO?
(i.e. all data including but not limited to Segmentation data, Campaign data, Lead data, opportunity data, Company data, Templates, Images, Reports, Lead Activity history / Tracking data history)
What exactly do you want to do?
You can export all lead data with a smart list and XLS export. It may take a while. Opps and Company Data live in your CRM.
If you really want your behavior data, then go to Admin > SFDC > Sync and let Marketo push Tasks for specific activities. Then use SFDC Campaigns to record Program data. Otherwise all of this is (and should be) in Marketo only.
All of that other data is tied into the Marketo system and would be lost if you left Marketo. Marketo has backups and redundancies, so I wouldn't worry about that.
I would just access the templates and copy/paste the code to local files. We actually do that today: we have local copies of every template stored in Dreamweaver. This also makes it convenient should we want to make any changes to a template using Dreamweaver (since there are no editing tools in Marketo for email templates).