Consider who you're teaching to. Most five year olds could care less about Van Gogh, and most adults wouldn't like to finger paint.
Consider what supplies you have on hand. Use that first before buying new supplies. If you have a lot of paint, use that.
Encourage creativity. Take the class on a walk, have them find something interesting to draw and have them draw it from memory. Have them research an artist and do a drawing in that artist's style.
Encourage even more creativity. Have them make a new dust jacket for their favorite book or movie. Take it a step further by only letting them use magazine clippings.
Have them all draw a picture based on the same model or reference picture. Hang all the pictures in the front of the room and marvel at the differences.
Have them draw self portraits, their favorite animal, what their house looks like, etc. Encourage recollection.
If you're teaching to a younger class, maybe use pipe cleaners and have them make models of animals, cars, people, etc.
Clay is also fun, but messy. Bowls and vases would be an interesting project. Kids can paint them or whatever and give them to their parents maybe as a Christmas/Parent's day present.
Cards are fun. Use colored paper and stickers. Have kids make them before holidays for teachers, parents, or each other.