I heard that the wine and beer was not that good
I don't know where you've heard this, but the beer is mostly Mythos or Amstel and is fine if you like normal lager, although if you're used to real ale or export strength lagers, it may not be to your taste. Greek wine has improved tremendously over the last 10 years or so and compares well to the average UK supermarket plonk from California, South Africa, etc. Again, if you're used to drinking wines costing £10 or more a bottle, it may not be quite to your taste, but it improves after the first couple of glasses
The main two Greek specialities are ouzo, an aniseed based Pernod-like drink which is excellent drunk before a meal as an apperitif and Metaxa brandy, which is more usually taken after meals. In some restaurants, whilst waiting for the bill you may also be ofered a small glass of raki (Greek fire-water) or something that I've never found out the name of, but it looks and tastes like a thick brown cough medicine - I'd advise you to knock this straight back, rather than sip it!