Mileto is absolutely correct!
Just check that link the week you are going, and you'l find the information specific for your visit (if DLP uploads it in time at least, but they're usually really good about that!)
As for random characters, I must admit I'm not a character hunting specialist, but I do know where to find the people that really know a lot about character hunting: charactercental.net
If no one here can fill you in about all the places where you can generally find "free roaming characters" then check out that forum
All that aside, please keep in mind that free roaming characters meeting may not be the most pleasant experience. Sometimes it's really good, but other times characters get absolutely mobbed, people push their way in front of you, parents keep on shoving their kids in front of you. You need a lot of patience, and to be able to control your temper (I've wanted to shout at people several times to be honest, I haven't, but I've always managed to restrain myself so far). To be honest, I get really fed up with the entire mobbing and rudeness so I stick to the characters with queues, as the experience of meeting them is generally much more pleasant
If you arrive at the queue say 10 minutes before the meet and greet starts, you will only have to queue for about 5 minutes, so it won't even take very long to be able to meet that character.
Of course with rare and popular characters, lots of people will be queuing for them already, but in general with "regular" characters, it's not much of a wait if you get there in time