Yay! sounds so fun, have a safe and beautiful trip! I would say not to take the metro once it gets dark- take cabs. And try not to look too much like a tourist (maps, fanny packs, etc LOL) it will only bring negative attention to yourself. Get your money exchanged into Euro's before you get there (they charge extra to do it at hotels/hot spots and a charge is added if you use an American debit/credit card). I would give you a list of attractions that everyone says is a "must-see" but honestly I had the most fulfilling times just walking, eating and people watching. Just say a safe travel prayer and go wherever the wind blows! God bless!