cheap flights

Doesn't look as if there are any

If you don't mind going to Cologne, there are flights from there to Athens with German Wings.

Normally transavia.com starts to fly in April to Athens, but don't know if it's early April. They used to fly as a charterflight for touroperators, so you can't book directly with them, but you can via e.g. TUI or Ross Holidays (ask a travelagent of check the web)
Otherwise, the only option is to fly Olympic or KLM for a direct flight. Although my brother came to visit me this April and he choose Alitalia which gave a good connection via Rome or Milan and was the cheapest deal. Also the flightschedule was pretty good.
There is a lowcost carrier from Brussels, www.brusselsairlines.com, I flew with them Athens-Brussels for 65 euro one way including taxes, if you book in advance you get pretty good deals.