The Rule Of Three
A writing friend has a theory that it takes 3 ideas to make a story, and we believe this works.
Rummage through your old files and notebooks for story fragments, scenes and ideas that didn't go anywhere. Pick three that still interest you and combine them into a new, whole story. (For example, you could take the setting from one, a character from another, and a situation from the third.)
You may need to think about the ideas from different angles to get them to fit together. Stick with it for 20 minutes - the subconscious is very clever at finding links and patterns.
Dynamic Duos
Try out this tongue-in-cheek internet toy for generating crime-fighting partnerships. Keep clicking until you find elements that appeal to you. Take as much or as little as you want of the character descriptions. Write for 20 minutes: a scene, some dialogue, or a plot outline.
Odd Man Out
Take one of the characters from your current work and write them an unconnected scene or short story. It may be helpful to do this for one of the characters you like/understand/know the least.
Picture Seeds
Use any or all of the pictures below as a starting point and write for 15 minutes.