The trip to the mountains turned out to be _______ than holidays at the seaside.

The correct answer is more exciting. Explanation: When comparing two things (a trip to the mountains and holidays at the seaside), we use the comparative form of an adjective. For adjectives with two or more syllables (like 'exciting'), we use 'more' before the adjective to form the comparative. 'More exciting' correctly compares the two experiences.