(follow-up) Well, for sure you will see a few young people conked out from drugs (usually not alcohol) here or there in the city. That is true even in Syntagma Square, the main square in town. But usually they are keeping to themselves, not dangerous. And pickpockets (when I was living there, they used to say, "It's those damned Albanians" -- blame the Albanians!), they are around (just like in Rome or other cities in Europe); just don't count money in public, etc. If you are in a group of five or something, stick together. Also, dress inobtrusively (that may be a challenge when on vacation), that is to say, don't stand out. The pickpockets are usually kids about 11 or 12, so you are not in physical danger. To your second question, Omonia Square is not far from the main action (Plaka, Syntagma Square and so on), less than a mile away, so the location is fine, distance-wise. Getting a hotel in Athens can be a major headache, so the sooner you make the reservations, whether at the Novotel, Astor, Plaka Hotel, or wherever, the better. You can do this!

It's about 10 minutes walk or so. It's an upmarket area with high end shops, bars, and restaurants.

You could walk from Plaka to Kolonaki square... it's a nice walk in the center of Athens