Consider ways to make activities easier or more challenging for different levels so you can reuse same activity for different classes. For example, if you are planning a dictation to help the students practice sound/spelling correlation you can have 3 different options. The easiest option can have students simply circle the words they hear by choosing from 2 options. The next step up would be a fill-in the blank with about 50% of the words missing. Finally, the most challenging version of this activity would be to have students write down the dictation word-for-word with no support from the teacher. You can use this same idea when preparing for a mixed level class.