Programming

Try making a game. I think minesweeper might be a good one because it can be broken down into many different problems with different complexities. Like user input could start out as entering coordinates into specific cells to interact with a seperate grid, then you could switch to using an input grid where you enter a value into the cell you want, and then move on to clicking on cells like in the real minesweeper, including different behaviours for left, right, and both clicks.

Pretty sure you could implement a full version of minesweeper in excel, though even if you can't get all the way, there should be enough low hanging fruit you can reach to learn a lot from the process.

Or if you're feeling really ambitious, I think a realistic physics racing simulator is also possible, though I wouldn't expect a lot on the graphics side of such a thing. Just lots of formulas that then get used to simulate a car accelerating, braking, and turning. But this one might also be great to get started with because you can start with a simple model and add complexity from there.