Gods, I have notebooks upon notebooks filled with ideas for inventions, schematics, blueprints, architectural drafts. Very hard to pick one. The most recent one, from today:

Problem: Rulers, the kind used in offices and schools. Currently: flat sticks of wood, with numbers printed along the edge. Takes up too much space, always hard to find the right size storage space, is too oddly shaped for everything else it gets stored with. And it's so restricted, it can only measure set amounts, can't adjust for larger or much smaller distances. Very primitive.

Solution: Very tiny, smaller than an eraser, device. Lay it flat on a surface, and it has a tiny appendage jutting out from the bottom that you place over the point where you want to start measuring. An additional piece (most likely fitted into a storage slot on the eraser-sized device for safe-keeping, and you can press a button to locate the additional piece if it gets lost)... Set it where you want to stop measuring. Press a button, and a display in the side of the eraser-sized device displays the length, in whatever units of measurements you choose.

You compensate for the two units being placed at odd angles by rotating the the top half of the eraser-sized unit until it aligns correctly with the endpoint set by the other device. Laser or similar line of light displays the path that's being measured.

This only works for measuring flat surfaces, of course. The usual kinds of surfaces you encounter in an office, at a desk, and related.