Adobe introduced some new versions of Creative Cloud today. And since not everyone upgrades right away, it's a great time to remind you how to save an InDesign file so it can be opened in an older version of InDesign.
Simply go to File > Export and choose "InDesign Markup Language (IDML)" at the bottom of the dialog box. The resulting file will be have an .idml extension and should open up in older versions of InDesign.
Of course you'll still need to package your images and fonts if you are transferring the file to another computer. Just collect the job by selecting File > Package... and selecting your options. Then save an IDML file, too.