Milk crates zip-tied together on their sides so that a bench seat fits on top and red bins from Lowe's can slide inside them. I don't have any milk crates, though. I also don't know what "zip-tied" means. I would use different colors.
Classroom B-day idea. or just a great community builder mid-year. This is seriously one of the sweetest ideas. If there is no smartboard in your room maybe you could do it with large bulletin board paper. Then they could keep the kind words.
Sub Tub -- Possible things to include: A Welcome Binder (explaining the daily schedule, where things are in the room and anything the sub might need like computer log-in codes), emergency lesson ideas, great books, games and other activities to do!
(Team Building) STEM Activity: Marshmallow Building Challenge - Give each group of kids 20 pieces of spaghetti, 1 yard of tape, 1 yard of string, and 1 giant marshmallow. Whoever builds the tallest structure wins!
Collaboration Activity (good for mentor time) from: In the middle: Life as a seventh grade language arts teacher. The Cup Challenge - teach students how to work in cooperative groups with this simple and fun challenge.