I would cut 2 days off Athens and 1 off Venice...those are really better as *one day* places to visit IMO and I'd add to the Greek Isles. Mykonos is fun and Ios I've heard is really good as well...I think the amount of time you have for Barcelona is good, if not less time actually...personally 5 days was a LOT of time in Madrid for me, but then again I tend to go crazy and walk all day seeing all the sights in like 1 day/2 max. But still, there's 3 day trips you can take from Madrid such as to Granada, Segovia and Toledo so I'd keep the extra days in Madrid as opposed to Barcelona, even though Barcelona does rock...(Gaudi is pretty much my idol, even though I dont want to become an architect at all...)
But your trip sounds amazing! Have fun!
If you walk backwards, you'll find out that you can go forward and people won't know if you're coming or going.
Dream as if you'll live forever. Live as if you'll die today.
Countries I've Visited: Austria, Botswana, Canada, Costa Rica, Democratic Republic of the Congo, France, Germany, Greece, Hong Kong, Italy. Malawi, Mexico, Portugal, Rwanda, South Africa, Spain, Taiwan, Tanzania, Uganda, USA, UK, Vatican City, Zambia