When a language isn't strongly typed, I often test unexpected but possible types -- for example: Numbers or Strings instead of Dates or Booleans. 

Other favorites: 

• ""
• "null"
• "undefined" 
• Quotes, comments, and other injection attack patterns. 

As Jon Willeke suggested, I also use random data when possible (apart from the -1, 0, 1 defaults).  The %Library.PopulateUtils class can be helpful for generating other preformatted random information. 

The version of the documentation should be clearly displayed as there are differences between the respective versions (and not just in appearance). 
The ordering of the sidebar is confusing to the point that I find it distracting.