That is how I do most of my math instruction. Monday and Friday are both whole group where I teach all on level concepts. Then I take Tuesday - Thursday to do small groups. I have groups A, B, and O. (Approaching, Beyond, and On) The kids don't know that's why I've called them that
Anyways - One T,W, or Th, I do a mini lesson for about 15-20 mins. Then I have three activities planned. One is always with me, one is always some kind of seat work, and the last is a fun game or activity. Each group will have a chance to do each activity. We do them for about 15 mins and then switch.
Then I always plan my group (the teacher group) by the ability of the groups. I'll do remediation with the low group, review with the on group, and extention with the above group. It works out great!!!
I think this would be great for an observation because not only will it show off instruction and differentiation but also classroom management.

Good Luck!