There are lots of great answers already, I'll just throw out one more thing: you could check around for a cycling skull-cap with an SPF rating. One example is this one from Giro, rated at SPF 30. It's designed to go under a helmet for summer use, and has an SPF rating.
Presumably any fabric will keep some sun off and could therefore have an SPF rating, but since this one actually provides the rating, you can compare it to sunscreens you've used to determine if it's sufficient. In my experience, SPF 30 is plenty for most conditions.
I haven't used that Giro cap myself, but I did use some Pearl Izumi SPF arm "warmers" which I actually found cooler than riding without sleeves. I used them on tour, sometimes doing 6+hours in the sun without sunscreen on my arms, and never got sunburnt while using them, whereas my back would sometimes get mildly sunburnt through my wool jersey.