Nearly any microcontroller project does lots of things "seemingly simultaneous".
things like:
* Keyboard scanning.
* Display output (muliplexing some 7-segment displays is a fun beginners project).
* Gathering sensor data, keeping track of limits etc.
* Communication with external devices.
And for all "simple" applications this is usually done in a master loop combined with neatly placed ISR routines.
For small systems an RTOS is not needed.
When projects get bigger, quite often an RTOS is still not strictly needed, but it can be nice to add that extra abstraction layer for code readability and re-usablility. Barriers between parts of the software get bigger, and apart from the inherent complexity of the RTOS in itself, this makes software easier to understand, maintain and extend.
But keeping motivation is also much easier when you have a project that personally interests you.
I think it may be a good idea to search around the net (and/or search directly on gitlab) for a project that interests you, and then re-write that project to work with an RTOS.
Also, reading and analyzing code written by others is a separate skill. It's often less fun then writing your own code, but it is a quite essential skill that enables you to stand on the shoulders of all the giants before you. Without that skill you're playing among the insects in the grass. That can be fun too (I remember some book about that). It's just another branch in this Wonderfully Weird World.