Something I have wanted to do is take an old laptop, convert it to a desktop, create a case for the screen and motherboard, wall mount the case, add a device like a leap motion for touch controls, and use it as a smart home control panel and/or a fancy weather clock with apps installed for sticky notes, photo album, etc.

Unfortunately the newest laptops you have are just a generation shy of meeting the minimum cpu requirements (AVX instruction necessary) for the leap motion in particular. But there may be alternatives. Hopefully the suggestion helps you think out of the box with repurposing older machines.

I also suggest turning them into emulation boxes.