#1. The Park Bench
This is a cliche that you see in every writing class but it's a goodie. The idea is to put two characters that you've been thinking about on a park bench together and get them talking. Yes I can hear you screaming from here, "No one sits and talks on a park bench anymore! I don't even know where I can find a park bench!" Chill out the idea is to find any public place where it's believable that two people could bump into each other and get them talking. Once you've made a commitment to the setting get them together until you've run out of things for them to say and then find a believable way to get them to leave.
#2. The Elevator Play
This is another cliche. In this one you put your characters into an elevator and have it shut down. As they're trying to get out and fear their imminent demise they get to talking about things. The idea is to get people in a situation where your characters can't escape and they don't know what's going to happen to them. This can take place in a myriad of places. The scene ends with the characters being allowed to escape.
#3. The ABC's
This might sound like a strange exercise but try it. Get out as many pages as you want your play to be and get as many characters as you want. Then starting with A have each line that your characters say start with a letter of the alphabet. When you get to Z start over again until you get tired of it. You'll be surprised that by the end of it that you have a good scene that you can edit later.