+1 on the cardboard boxes.
You didn't mention the age of your target group, but I'm assuming early elementary age.
My daughter (almost 8 years old) just got started playing around with an Arduino for her school's science fair project. Obviously, there was a healthy dose of coaching on my part (but since you're talking about dedicating 2 days a month to the project, that shouldn't be a stumbling point for you) regarding selection of resistors, etc., but the programming was something she was able to work out on her own. We used Scratch (the visual programming language) and the
Scratch 4 Arduino firmware. The
S4A workshop is a great place to start if you do not have prior experience with the platform.
I started the process by explaining what a program is (i.e. a series of well defined, discrete steps that lead to the desired goal), and, placing a ball on the floor in the middle of the room, instructed her to pick it up by going through a program (e.g. walk three steps; take one step to the left; walk three steps; bend; extend arm; grasp ball). This was sufficient to get her started. YMMV.
I'm working up an introductory program for the teachers at my daughter's school, as they would like to start using Arduinos in the classroom next year. As I develop the program, I'd be happy to post the results.
One caveat. I had purchased an introductory Arduino kit from Banggood. This kit comes with breadboard, leads, servo, stepper, LED matrix, buttons, LEDs, a mix of resistors, etc. Why the kit is a bargain for what it contains, I would recommend
splurging on a higher quality breadboard and leads. Far and away the most frustrating part of the entire process for my daughter was fighting to insert low quality leads into the holes on a poorer than low quality breadboard. (Also, the resistors that came with the kit were the relatively arcane 5 band type, rather than the easier to decipher 4 band sort.) Since a high quality breadboard and a bundle of good leads of varying lengths will set you back about 10 bucks, there really doesn't seem to be much advantage in burning the kids out on a struggle with imported garbage.
Good luck on what sounds like a very rewarding process.
--Josh