Link to PSet
Link to Answer Key
Here is the first proper Bespoke Mathematics Problem Set! In these questions, you'll meet up with some very fun shapes called "polyominoes" which are a mainstay of combinatorial geometry. There's a TON of material about polyominoes out there and if you've played Tetris, you already know what these things are.
This problem set is mostly about teaching kids how we pick a definition for a new class of mathematical objects (establishing the "rules" for polyominoes) and getting them to play within those constraints to make new things. One thing I wanted to emphasize here is that it's okay to try out different rules, but being consistent once you've decided how you want things to work matters a lot.
For a great introduction to polyominoes, I can't think of anything better than the original Martin Gardner article about them. You can find this in the collected volumes of his work (which are unbeatable) published by the AMA. Alternatively, many places online have nice articles about these - try Wolfram or www.mathpuzzle.com for some pointers.