@jjorge,
Hi jjorge. I think it's wonderful that these children will have the benefit of your experiences as part of their school experience.
I've taught as the regular hired teacher and as a sub and the two can vary wildly- but in the main, I've enjoyed both.
I would say that having the class be your own is usually easier, but being able to move around and substitute can often be more interesting- depending on the ages of the students you're teaching.
First of all, if the classroom teacher leaves a plan or activity, it' important for you to try to do that with the class if you want that teacher to request you again.
They've probably gone to some effort to put that activity and the sub plan together and would like to see that effort respected by having their plan implemented. So I'd start with their plan first.
But I do think it's a good idea to have a back-up plan of your own in case you find that you finish way early with the teacher's lesson plan.
There are a lot of good websites with interesting and distinct one-lesson activities.
I've made a file of these in both literacy and numeracy to have on hand. In the situation in which I teach, I can be asked to teach literacy or numeracy from entry level (which entails recognizing the letters of the alphabet and being able to count) to Level III (which is A-level over here and 10th grade over there and can be anything from analyzing poetry to solving proofs in Geometry).
Because I have developed this file, when I am asked to abandon my plans for the day and fill in for someone else, I have activities that I'm familiar with and that I know I don't need a lot of prep or materials for - as long as I have paper, pens, a whiteboard and participating students - I'm good to go.
Yesterday I was asked to cover a class five minutes before class time. There were no sub plans as it was an unplanned absence. The students were Level I and II Literacy students.
We did an activity on Respect/Adjectives. Each learner had to come up with one name in categories that included politicians, writers, artists, musicians, human rights activists, etc., etc. - give one adjective that they thought described their candidate, and write an essay on someone in their life for whom they feel respect and what about that person has encouraged that respect.
It was a great discussion - we came out of it with a writing sample to give their teacher - and it was interesting and stimulating for me - I learned a lot about writers, artists and musicians I didn't know.
I do tend to like working with older students. I have MUCH respect for kindergarten and grade school teachers and substitutes. That can be very exhausting. Just be conscious of your energy level and make sure you're up for a seven hour day with twenty-something five year olds and all the unplanned and spontaneous issues that can arise around them.
(I say that with a smile).
But I like moving around and covering other classes...you get to meet a lot more interesting people that way.
Good luck - I hope you enjoy it.