What can I use as proof of travel history to apply for a Canadian visa?

I agree with @Mark but suspect whatever evidence you provided would have made very little difference, if any. Visa authorities rarely if ever ask for what towns and cities, or villages or neighbours, applicants have visited within their own country. What they care about are border crossings. Hence passport stamps that show these, and visas that show permission to apply to cross borders. Hence their your duration outside of your country and to some extent boarding passes, since these are usually only issued for long and therefore potentially transnational journeys.

Canada, like most other countries, does not want unemployment amongst its own nationals to go up because of work performed by non-Canadians in Canada. It also does not want Canadian tax payers to pay for any more welfare support than the Canadian government deems necessary. So it checks potential visitors to reduce the chances of either. A history of conforming, even if in other countries, is taken as some guide to likely future behaviour and, since other countries have much the same objectives, permissions from them are reassuring. A prematurely cancelled visa is a sign of issues, so a visa that has not been cancelled at least is better than that.

In your case you mention that your travel did not require even a passport so would have proved nothing of value even had you submitted comprehensive documentation of it. Where there are no limits on your travel (time or distance) nor on activities (tourism, study etc) then such travel establishes nothing significant about the chances of your complying with limits where those do apply.

In effect, you had no travel history worth showing so the answer to your question is "nothing". As you know from your fiancée's experience, that does not mean to say a visa is sure to be refused, but it is likely to reduce the chances of being issued – as you have found from your own experience.

You did the best you could and I'm sorry you did not make it to Canada (somewhere I also would like to visit). You were both higher than average risk (university student, not married, Romanian) so maybe consider that your fiancée was lucky rather than you unlucky.