INDD files cannot be opened by any other application. Nor can they be opened by an InDesign version earlier than they were created in unless (when you had a subscription) you saved them as an IDML file which can be opened by InDesign applications back to CS4.
If you had a subscription, you could also make a PDF file from them so you could read them.
The only solution would be to (1) use another computer, and install a 7-day InDesign trial. You cannot install a trial on a computer once you've installed the program. Or (2) purchase a one-month subscription to InDesign.