“ I decided to take a spontaneous trip to Belize. I didn’t bring a suitcase or anything. I just went to the airport and hopped on the first flight they could offer me!”

Spontaneous trip is correct :) Other ways to say it “an impromptu trip”, “an unplanned trip” (not as strong with the meaning though), “a spur-of-the-moment trip”. I wouldn’t say spontaneously fell asleep in speech. I’d say “nodded off”, “drifted off” or “dozed off”.