Coffee shops are one of my personal favorites when it comes to places to meet singles. It may seem obvious to some or completely alien to others - but it works. Find a busy coffee shop that you feel comfortable in, and take up a space near the main door. Don't bring anything to distract yourself with if you can manage it, although this may be more difficult for some than for others. The point is this: try not to involve yourself into something so completely that you send off 'I'm busy' vibes. You want to seem approachable, and hiding behind a laptop or an book doesn't help!
Then, make a point of 'greeting' each person who comes in the door, whether with a smile, comment or friendly hello. There is no need to make your introduction rote or sounding like you're a Walmart greeter however; just try and be yourself doing something that may not come naturally. By doing this you are not only making yourself known to all of the other patrons, you are also making yourself available to anything that comes your way - which could be a recommendation for a new book to read, a compliment about something you're wearing, or maybe even a date. Worst case scenario? Someone doesn't say hi back, and another person walks through the door ten seconds later. You've lost nothing in the process, and have opened yourself up just a bit further.