Make 5-10 of multivibrators each on different schemeatics. Bonus point - make them as fast as you can - starting from the prototyping stage and finishing with the device ready to be either gifted or used as a lab generator.

But playing with clocks and multiplexer is definitely not a beginning of the ladder.

Yeah, it seems clocks and muxes is what comes after this book? A possibly not so great suggestion is building a radio(receiver)? It is pretty challenging with oscillators, tuned circuits, mixers, amplifiers (basically a mix of RF, AC and DC).