I agree with buying a longer pass, the cost per day goes down a bunch with the longer passes and not having to rush to see things is worth it.
I also agree with getting to know some of the "lesser" sights that are free with your pass. With a pass, stopping in for a quick look around makes sense, without a pass you would skip them. Using them as a source for good clean toilets is a good idea.
I'll second St. Denis. I'm a history buff and the sheer number of French royalty buried in the crypt is amazing.
Not so much for Vincennes. There is very little to see. The keep has something like ten empty rooms to look at. The chapel is small and also empty. There is some view from one side of the castle - and that's about it. The rest of the space is used as archives/storage for French government agencies and not viewable for the public. I think some people are enamoured by it's connection to Marquis de Sade. For me, it wasn't worth the time spent to get there.