I shall be in Athens on Weds , I'll give an update on my return .. my guess is that you'd need a couple of days to do Athens justice ...
you could consider a day trip to Delphi as well but it is about 3 hours each way by bus and some of the day tours don't seem to give you very long there , so it might not be worth it
Zante is only a 55 min flight from Athens so not what I would deem a lot of hassle , about £50 each way ... if you initially get an Easy jet type one way flight to Athens you could then get a direct flight back to the UK from Zante for about £140 (into Luton or LGW ..didn't see a BHX flight) , save you the hassle (and £50) of going back to Athens
You need to set up your own website/company Tony cheers for that man.
Flights to Zante are available from all over - Monarch do a BHX I think, Leeds Bradford with Jet2, Manc with various, EMA with loads all around etc - accomodation is cheap on Zante also and you can get 1-5 star whatever you want .