Friend, you don’t need an SDMB thread, you need a book – and there are plenty to choose from. You could spend your entire time in only one of the major countries and not run out of must-see stuff – nevermind the entire continent. I hope you have a blast doing it.
(What, me, a Europhile? Naahhh…)
I’ll second this thought. Making churches the focus of the trip would probably be boring (if you are not an architectural student), but there are a few that are amazing works of art, regardless of one’s thoughts about religion.
As it happens, one of my favorites was the Kölner Dom, (next to the railroad station), with an austere but very beautiful interior and a steeple that provides a marvelous view of Cologne.
In Paris, the Sainte Chapelle at the Palais de Justice is a tiny chapel that is best visted on a day with hazy bright sunshine. Entering through the almost oppressive crypt-like lower level with small windows that are a dark violet, one climbs the stairs, stepping into a world defined entirely by light and color where its walls, (nearly entirely stained glass windows), simply glow with almost psychedelic color. (The linked photos do NOT do it justice.)
Outside Paris, the Chartres Cathedral is deservedly renowned for its stained glass windows. Later cathedrals were able to use more advanced techniques to create windows with finer filigree, but few approached Chartres for sheer artistry.
I will not get into a pissing contest over whether the French or Spanish Riviera is “better,” but if you are going to Barcelona, a trip by boat (operated by the Spanish Rail system) up the Costa Brava to the North of Barcelona provides a number of beautiful small towns with great beaches. (Not much surf, but no one goes to the Mediterranean for surf. 
) (Obviously, if you are driving, the towns are still accessible.)