digital circuit idea

I have been creating various analog circuits for the past few months, and have decided to try digital circuits. I am looking for a circuit that is challenging but not too hard, if is not too much to ask i would prefer it be somewhat useful, e.x. a circuit that stops charging a capacitor after a certain amount of time. I dont care how long it takes just as long as it is somewhat challenging and useful. Thank you ahead of time.

Try some basic flip flops, digital circuits aren't very interesting until they start to get complicated there are more than enough online examples to get you started if you just search for a bit using Google.