Strimko is an interesting take on a Sudoku-like puzzle. You fill in grids with the numbers 1 to 4 (at the start, at least) not having the same number repeat in a row or column, nor in a "chain" of four numbers linked by white lines. It sounds oddly simple, but is in fact immediately a lot more tricksy than an easy sudoku, for instance. And there's a demo of it you can try.
At first it didn't click, and I thought it was closer to the tiresome old number grids where you had to find a way to insert all the numbers through blind luck. This is partly the fault of the game's own tutorial, which completely fails to teach you how the game should actually be played. It is, instead, about using the (unmentioned) smaller discs surrounding each circle to put in the potential numbers that might fit there, and then from collecting enough of these being able to eliminate options until a number becomes inevitable somewhere in the grid. Then of course once that number's in place, it changes the conditions for all you other plotted potentials. So much like solving a medium to hard sudoku, for those familiar with the techniques.
It's extremely peculiar that the tutorial doesn't think to mention any of this, instead teaching you how to play on a 3x3 grid, then starting you on a 4x4 for which completely different approaches are required. It's almost as silly as the gibberish that surrounds the puzzles, which is something to do with gathering scattered parts for a tool that can reclaim a planet destroyed by blah blah blah, and then some guff about astrological signs and so on.
However, that's all easily ignored, and indeed should you wish to escape it entirely there's a mode for just the puzzles. The challenges also change as you get further in, with new twists and rules appearing. And as a puzzle, it's a good idea, and I think more immediately interesting than the more famous 9x9 world conqueror. You can get the demo from here, and read more about the game here. Sadly the demo is a measly 45 minutes. They couldn't round up to the full hour? Meanies.