Eurotrip through France, Spain and Portugal?

My friends and I are planning a Eurotrip beginning in France, going through Spain and ending in Portugal. We want to get a taste of everything- the countryside, the mountains (preferably the Alps), the beaches, the cities- but we only have twenty days! What are some key spots we should hit? We were thinking about starting in Grenoble, moving south, traveling along the coast till we hit Spain then exploring for a bit before heading to Portugal. Anyone done a similar trip or have any suggestions?