I just finished lights around a constant combinator that serves as a switch for recovery of power in case of a brownout on Vulcanus. The lights are circling around the combinator while changing colours through the whole spectrum.
It’s completely useless and I love it and am very proud of it. I spent 2 evenings on it :-)
Some basic circuits are quite simple, if you hooked up an alarm, there’s a lot you can do. For example, controlling inserters based on chest contents can be very helpful when you output multiple products into a single passive provider to avoid one product taking all space. Or observing a single accumulator charge to disengage power switch :-) But the most complicated logic I did is really only decorative. And I’m also having fun with multiproduct assemblers.