I always volunteer at Feed My Starving Children.
You are near the Twin Cities? I've been there twice...
Anyway, as to the OP
Contact your local Chamber of Commerce or umbrella non-proffit organization to get a list of the local non-proffits. They'll be able to point you to many different locations. A back-up might be going through social services or the judicial system for service hours. The latter sends out many folks to fulfill their obligation to society and might have some contacts. Many states or local governments are also in need of labor.
Otherwise:
Schools (after school programs, supplimentary instruction, photocpies, etc.)
Churches
Food banks
Local Government (parks and rec for helping out with programs, facilities upkeep, park clean-ups, etc.)
Animal Shelter
Human Shelter
The place where I work would be unusable if it wasn't for some volunteer hours spent on maintenance. With my wife's work, many folks would go hungry if it wasn't for volunteers.
Good luck!