Trip Advisor and Lonely Planet are always great places to start. They both have forums, and good research tools.
We went in 2003. The country was in big prep for the Olympics, so I'm sure lots has changed since I was there. We stayed at the Hotel Ledra and the Royal Olympic Hotel.
The Ledra was AWESOME. It had a rooftop pool and bar, with great views. It is quite far from the city center, but it did have a little van that would drive you there. They were building a subway line right next door, so it's totally possible it's done now, and very easy to get into the city center. I think the hotel is one of the more expensive in Athens, it's run by Marriott, and part of our stay was on points. But it's a very luxurious, American style hotel (akac comfortable matresses, and nice pillows
)
The Royal Olympic was also quite nice, although a Europen style hotel. It was in a fantastic location, right across from the Temple of Zeus and easy walking to the Plaka and other tourist sights. I have no idea of the price, it was part of a tour we were on.
I could have spent many, many days wandering around the Plaka (shopping district, but old school.) Tons of funny little shops and soulvaki joints etc. My favorite spot we visited was the Agora (borders the Plaka), I just liked the little museum and the temples there.
However, if you can manage it, I would really suggest getting out of Athens. It's a huge city but honestly, you could see pretty much everything you could ever want to see in about 3 days. We travelled around the mainland and saw some pretty cool things (happy to suggest some of those), but the islands were amazing as well. You can catch ferries from the Athen's port town Piraeus (maybe a 20 minute subway ride). It is well worth the effort. Santorini is just beautiful and Mykonos is quite fun as well. There's a ton of places to check out.
