Here is an activity I use to encourage the students to work through some question in class.
It does require some initial monetary outlay, but it can be used for a very long time.
You will need some sort of locking mechanism, a padlock or safe, that requires a three (or four) digit pass code to unlock.
It should be visible to the class, It works better than a virtual one, although if you could find a digital one professionally done then that might be quite interesting.
Have a prize inside, and sometimes wrap it in an extension activity, in case they crack your code early.
I have a lock which can be reset to different combinations, you can get them for under £4 from Ebay or Amazon. I have also been trying to find a safe deposit box that works with a padlock, but yet to find one, so I bought some chain links from Homebase, for about £2. I then wrapped it round a normal tin and this is what I get:
The worksheets are then designed to give three numbers which relate to the code. I do not check their work until after they have tried the code. If a student tries to guess the code, we can have a discussion on permutations and chance, and the only rule is that you have to wait three minutes before your next guess.
The worksheets cover many misconceptions as they include true or false statements. So some are straightforward, whilst others can encourage debate.
Of course, locking the safe back up so other finishers can guess the code is fairly straightforward.
Those that finish first are asked to note down in their books three key facts that helped them solve the code, and then they can support other teams. So far I have seen relatively little cheating.
Here are the link to some sets of questions, starting with percentages and fractions, I will add more as I do them in lesson. Please feel free to create and share your own.
I actually think you could use a plastic bag as a safe: Put a box in the bag that is big enough so that if the bag handles were locked together, you couldn't get the box out. Sure, you could rip the plastic bag, but if you have nice students I don't think that would matter!