Can't "interesujące" also mean interesting? My Polish teacher always uses it, but Duolingo marked it as incorrect
That was because of the form. "interesujące" is either neuter singular or not masculine-personal plural.
"podróż" is feminine, therefore: interesująca podróż
"podróż" is female?
Yes. Count it as an exception, although of course there will be more of them.