Unique Identifiers Across Multiple Surveys

Hello,
I'm conducting an online experiment and embedded multiple Qualtrics surveys in an online authoring tool. This was done through iFrames. The tool leads people through various stages, so when they progress and are exposed to new information, then they complete the surveys.
Because data collection is anonymous and the individual surveys won't recognise past completion (as each survey is set up individually and come at various stages), how could I best setup unique identifiers? Query strings don't seem to be an option as the survey links won't be mailed out. I'm thinking about IP addresses or having participants enter their own identifier, but ideally I've something automated behind the scenes.
Any thoughts I'd greatly appreciate it!