A few weeks ago I was waiting for a friend so we could catch up at Cafe Latte. I spent my time sketching people in line at the counter. This is a great opportunity for getting practice with hairstyles and clothing. The patrons are so interested in making the meal choices (and dessert choices) that they don't look around and notice that anyone is watching and sketching!
Use those moments when you're out and about waiting as opportunities to practice, notice, observe. If conditions are right you can also overhear conversations and learn what's on everyone's mind, and what's "trending." It's a great way to past the time.