Ring that cab: If you are going to take a taxi, your best bet is to book a taxi by phone rather than take one from a rank. More accessible, and the chances are they won't overcharge you.
Tour it all: Book a tour that ensures you see the top places like the old town, the monuments, castles and the cathedral.
Good to know
The summer months between May and September are the best times to visit. This is because it's often pleasantly warm, whereas the dark days of winter can get very cold!