After experimenting with Office 365 I ran into this really useful tool availabe called NAPA Office Development Tools. This tool allows you to create Apps for SharePoint and Office. Please refer to the following NAPA tutorials for more information.
Outlook Mail app – http://msdn.microsoft.com/en-us/library/jj220072.aspx
Excel App – http://msdn.microsoft.com/en-us/library/jj220065.aspx
SharePoint App – http://msdn.microsoft.com/en-us/library/jj220041.aspx
Please be aware that when you create an app for SharePoint it creates a sub web. If you are interested in accessing any lists on the top site level you need to provide permissions through the properties window in NAPA Application. I will blog about this soon.