(1) a great holiday: because it describes a specific holiday (your current one)(2) the Colosseum: because it's a specific and unique landmark(3) a fantastic museum: because it's not a specific one you've mentioned before(4) an ice cream: because each is separate and unspecific(5) the city center: because it's a specific landmark within the city(6) the best ice cream: because you're making a general statement about all ice cream(7) the world: because it's a specific thing (the world)(8) the Trevi Fountain: because it's a specific and unique landmark(9) a palace: because you haven't mentioned which one specifically(10) a really wonderful city: because you're making a general statement about your personal experience of Rome