SharePoint Online (Office 365)
You can "Sign up for an Office 365 Developer Site" which will cost you $99.00 per year for an Office 365 developer subscription.
You might get one for free if you are a MSDN subscriber or a (Plan E1 or E3) Office 365 subscriber
You could also become a Microsoft Cloud Essentials Partner which will get you 25 seats of Microsoft Office 365.
As Luis said, please note that there are important functional differences between SharePoint Online (part of Office 365) and a SharePoint 2013 on-premises installation. So in order to give a full demo, you could set up your own server using the free "SharePoint Foundation 2013" version or "SharePoint Server 2013 - 180-day trial (with enterprise features)".
There is a ready to use template on Azure that you could use: Get Started with SharePoint Server 2013 Quickly on Windows Azure
And then there also are the Contoso images...