"Was your trip to the US fun?"
2020.5.3 Yeah noun (as adjective) + noun can often drop the の。Japanese is flexible like that. That's why so many particles can be dropped in conversation, as long as both the listener and speaker understand each other, a OK
There's a famous Japanese proverb, 以心伝心「い。しん。でん。しん」, which loosely means "of minds to communicate by thought." That's how the Japanese feel they communicate because so many things are assumed by context, not by explicitly stating it.
When the context gets lost, then they'll start asking questions to narrow down and reestablish the context.
From speakers of a language that explicitly states everything, this is a bit hard to wrap their heads around, and believe one can communicate accurately like this.
But both Koreans and Turkish speak this way too