You can tackle so many projects with a Raspberry Pi that it becomes overwhelming if you don't know where to start. If you're feeling nostalgic, you can attempt to recreate an early Macintosh computer running Mac OS 7, an operating system that dates back to the early '90s. Don't expect to get a replacement computer out of this Mac recreation, but instead, treat it as a fun side project.
If you're interested, you'll need to pick up a Raspberry Pi 3B+ or earlier, as this project doesn't support later models. Next, you'll have to pick up an old Macintosh Classic. Your best bet here is to scour eBay and see if you can find a broken one for cheap since it doesn't matter what shape the components are in. From there, you'll want to strip the Mac of all its parts, but ensure you keep the housing intact. If you like to tinker, the computer's guts will be nice to keep around, but they're not essential for this Raspberry Pi build.
How to transform a Mac into a Raspberry Pi project
There are many examples of people retrofitting an old Mac with a Raspberry Pi. The one in the video above from jaromaz will have you mounting an LCD screen with a Pi housed inside of an old Mac's shell. All original components, including the screen, get replaced in this scenario. You can save the screen if it's in good shape, but it's far easier from a compatibility perspective to just replace it. You'll get a better resolution with a new screen, but you will have to deal with the original Mac screen having the CRT monitor curve. A 3D-printed part will fix that for you if you're switching to a flat LCD screen, but that's only if you have a 3D printer handy.
Don't forget to grab software for the Mac OS you're looking for. Basilisk II is a popular option for Mac OS 7 and 8 if that's what you want. When that's installed, you'll have a new machine that not only looks like a classic Macintosh but performs like one as well. For more detail, jaromaz breaks down the whole build on GitHub.
What's the point of this?
Like many Pi projects, there's not much of a point outside of simply wanting to do it and having fun with it. A Mac from the '90s has been completely outclassed by anything modern, but this isn't a difficult project to create if you're feeling nostalgic. You can run some old-school games like "Quake 3" or "Marathon" depending on what classic Mac OS version you've installed. You can even create a retro gaming console housed inside a Mac's shell that runs old Nintendo or PlayStation games. If all you're looking for is recreating the look but not the feel, you can skip all the steps about installing a Mac OS.
Most of the nostalgia will come from the look, but if you really want to relive what it was like to have a Mac in the '90s, you'll want to install the old OS. For bonus points, you can pair the build with an old mouse and keyboard.