I really love teaching addition because it's easy to differentiate so you reach all learners. I love that once my quick learners catch on, I can give them harder numbers or an addition game to practice and pull small groups without having to worry about too many melt downs.
A couple years ago I created this worksheet, math mat and SMARTboard file to help my students practice addition. After we have practiced whole group, I give them a worksheet, math manipulatives and a math mat.
In the past I have used counting bears, bugs or dinosaurs for manipulatives. However, now that I have 37 students (!!), I have been giving each student a snap cube tower that has 12 cubes. I try to give each student a different color so they don't mix up their cubes. Once they're done with their activity, they put the tower back together again and into a basket.
While using the mat we begin practicing addition stories (Curious George ate 3 bananas in a tree and 1 banana on the ground. How many did he eat?)
After we have mastered the concept, I introduce our "addition game." I give them a blank worksheet and a die to make their own addition sentences. I never mention it, but it seems to become a contest to see who can get Curious George to eat more bananas! :)
We use this SMARTboard file with flashcards while we're waiting for the last few kids to finish or we're packing up. I also have this SMARTboard file that they love to use also! :)