There isn’t anything quite like Mobirise, unfortunately, in terms of ease of use combined with the ability to publish to GitHub Pages. (That’s why I chose it!) However, there are some alternatives you might consider:
Build your website by hand
There’s nothing wrong with building your website by hand, with HTML and CSS, and it would be a terrific way to learn the in’s and out’s of web development.
A site template
If you start with an HTML/CSS template, you can customize it by adding and changing elements with a code editor and a browser. Once you’ve created something you like, you can upload your site to GitHub pages. Here is a big library of HTML/CSS templates.
Publii
Publii is a tool for building websites that, like Mobirise, has a GUI (graphic user interface, meaning you can drag and drop things). I don’t have experience with it, but it looks very promising, with the added benefit that it’s an open-source, free-to-use project that allows you to connect directly to GitHub Pages.
Hugo
Hugo is a static site generator designed to be used with GitHub Pages. It is fast and free, but you need to use the command line and there will be a learning curve.
Purchasing server space and using a content management system
I know, this sounds daunting, but it’s actually not! A server, as we’ve learned, is just a computer that “broadcasts” your website to the internet. A content management system makes it easy to build a website, usually with a drag-and-drop interface. Most CMS’s store your text, images, and other media in a database, so that it can retrieve them in the order you specify. GitHub Pages does not allow you to use a database to store your content, which is why you’d need to purchase your own server space.
Purchasing server space is way easier than it sounds — I recommend Reclaim Hosting, which costs about $30 a year, including your very own URL. With one click, you can install a CMS like WordPress (the same CMS this very site is running) and customize it to your heart’s content.
WordPress.com and alternatives
If absolutely none of this sounds appealing, there’s one more option, and it’s the easiest one yet: You can use wordpress.com (or some alternatives listed below) to build your website on someone else’s server. You won’t need to worry about server administration or hand coding anything. The downside is that you may not learn as much as you would with some other options, but perhaps you want to prioritize learning other things. That’s OK!