Come up with a fake game and make assets for it. If you don't have a dream game, consider updating the art for a very old game, or demaking a 3D or high-res 2D game.
Making items in isolation won't teach you as much, and you'll just be lost for subjects. You could pixel a beautiful tree and a beautiful car, but if you can't make them look good together as part of a single scene, then what good is that?

While there are certainly some things pixel artists are usually hired to do more than than other things (stone floor and grass tiles and trees are much more common than train engine rooms, for example), most things really rely on the same set of skills, so don't be afraid to branch out and pixel unusual things, as those will still teach you a lot of useful techniques and principles.

Lastly, and this might be something you already know, but remember that there are a lot of skills that go into pixel art that aren't pixel-specific, such as colour theory, perspective, anatomy, lighting, etc. If you struggle with those, practice in a medium that's more familiar to you. It's much easier to focus on one thing at a time than to try to learn several things at once.