Today's Teacher Appreciation idea is quick and easy. Depending on the mug you purchase, it can also be a very cost effective gift if you have lots of Teachers to remember. Last year I found this cute chalkboard mug at Starbucks:
It was just begging to go home with me! I had each of the kids sign it with a white paint pen. It still looked like chalk, but was a bit more permanent.
Also, Ikea has plain white mugs for $1. They are a nice shape, and would work well if you needed to have multiple gifts for multiple teachers. That price is cheaper than most gift bags!