"If your feet are cold, put on a cap" is an old adage reminding us that insulation for your head is very important for your overall body temperature. The most common insulation is a cap or a beanie but there are some other options as well.

My favourite cap for three reasons; I like a beak to protect my glasses from rain and sun. I made it myself. It also holds a piece of cloth that can be used for protecting your neck and ears from the sun.
 Needless to say, not everyone has this need or inclination and some people go bareheaded and others wear bandanas or whatever. Up to you.
The fleece cap is basic and excellent, keeping my son warm during a break on the tundra. Does he look doubtful? Always looks at me that way...
Less basic, warmer and a lot uglier is the bomber type cap a la Ray Jardine. Homemade with two layers of thin ripstop and Primaloft sandwiched in between, it keeps me warm on a chilly, late September hike in the far north. And, no, there is nothing but tea in the cup...
Comments and new insights
For summer and fall trips I keep my head insulated by using, either singly or in combination, a merino wool undershirt with a hood, a windshirt with a hood and a rain jacket with a hood. The insulated bomber cap is for when I use my quilt. In my sleeping bag the merino hoody is enough. Furthermore, very often I use a warm jacket that also has an insulated hood. This means that I do not normally need a separate warm cap, like the one my son is using in the photo.