A website that talks about a hobby or special interest. Originality doesn't matter, as it will have been done before anyway, but that is an advantage in your particular situation. Use the ideas that you see on similar sites.
Design a site for a friend's business, even if they don't need one. Good practice and gives you a feel for commercial web design. Bung it in a sub-folder of your server space. If they like it and it's good enough, they might even want to use it.
Think up a music artist or band that you would be interested in and design a site that showcases them. Again, bung it in a subfolder and no-one need ever see it if you don't want them to.
Work out what you would like to say, even if it's just fantasy, then work out how to say and you have yourself a website.