Simple concept: A hat with a cup holder. I’m thinking: Would be kind of tipsy. So how about one on each side, with a weighted disc. Empty they’re both balanced. Got a drink? Move both weights to one side, put the drink in the other. Gotta have a long flexible straw, of course, so you can sip while you walk. Remove weights as liquid level lowers.
Maybe weights could serve a purpose: Coasters? Can openers? Coasters with can openers? (Hmm … why haven’t I seen those … a coaster with a built-in foldout can opener?)
Great for hands-free driving? Maybe too visible to police.
2016 UPDATE: Uh, this idea has obviously occurred to others, since I noticed examples not long after writing this post. Here’s a beer hat sold at Walmart.