In this amazing TED talk, Paola Antonelli, a MOMA senior curator, discusses criteria for the collections she decides belong in the museum. This is a great way to get your students talking about what they consider art to be. Several of the things they bring up may differ from your own understanding, but that's what makes it so fun!
These (free!) lesson plans by the Getty Museum are designed for all grades and concentrate on how art changes as it constantly inspires new generations. This is a perfect way to get your students accustomed to the idea of art as a vehicle for political and social change, and the site is a great primary resource if you're asking them to do research.
This page, by the Metropolitan Museum of Art, offers free print and online resources for instructors which are incredibly handy if you either want to incorporate one of their specific lessons, or if you are just lacking a sub plan and need to come up with something quickly.
Even if your class isn't located in New York, you can still look at MOMA exhibits online with your students or offer the site as a primary source to get them inspired! I often assign my students web pages like this to serve as their primary sources when they are creating a work of art that mimics their favorite style.