I found a wonderful activity to use for bellwork while I was student teaching. They are called "Drops in the Bucket" from Frog Publications. What it is is a daily review of Language Arts or Math. The key is that it reviews the skills from the previous year. Each day they focuses on skills such as: beginning sounds, homophones, antonyms, pronouns, geometry, measurement, money, time and many more. The students are able to come in each morning and do the work with confidence and feel success because they already have experience from the previous year. Also, you will feel confident that your class is doing a bit of review each day to strengthen skills needed to grow. They will not be able to forget a skill and will constantly reinforce what is already known. I also use it as a guide for mini-lessons that are needed for large or small groups.