I remember as a student teacher, my "short practicum" classroom was in a Middle School and the first 15 minutes of the day was "advisory" or "homeroom" It was a mixed class (grades 6, 7, 8) and a time to warm-up for the day and connect with students and staff that were not in your regular classes.
Some days there was computer lab time, some days was free play, some days was focussed activities.... What I loved about the teacher I worked with was that she had riddles on the board, every day... students had a focal point to think about when they arrived and later in the day she would discuss the riddle and see if anyone got the answer.
I found this on a website and thought I'd share it as a suggestion for doing something similar in classrooms you teach in as a TTOC:
Putting a worksheet on each student's desk, or an activity on the board before school starts, will get kids off to a great morning. This also helps prevent potential behavior problems as students come into class. To make it easier, have self-starters according to the following days:Monday - "Monday Math" (ex: put 5 math review problems on the board)
Tuesday - "Talk It Over Tuesday" (ex: write some quotes on the board for kids to edit for punctuations, grammar, etc.)
Wednesday - "Wacky Word-Wall Wednesday" (ex: put 5 of the words from the word wall in sentences, in tongue twisters, etc.)
Thursday - "Think it Through Thursday" (ex: put a thought-provoking question on the board for students to respond to in their journals)
Friday - "Fun Friday" (ex: put afun crossword, wordsearch, coloring sheet, etc. on their desk to complete "just for fun."
I really love the idea of theme days, but if you are a Teacher-Teaching-On-Call you could potentially do the same activity multiple days since you are in multiple classrooms within a week.
I find riddles, one-minute mysteries, and word games (liike Boggle) work well.
this discussion forum has more ideas for start of day activities