As a kid in the 70s, I only had a general Tandy electronics kit. Then soon a Vic-20. Much later I bought a kit from Busch with an LCD display (#2188).
Eds mentioned the great Busch 2090 kit.
-A 4 bit trainer from Germany - Busch Microtronic aka 2090
There’s also now an emulator.
GitHub - lambdamikel/Busch-2090: An Emulator of the Busch 2090 Microtronic Computer System for Arduino Uno and Arduino Mega 2560
I also talked about toy computers and the ITT MP Experimenter.
Another great computer from kit manufacturer Kosmos is the CP1 with membrane keyboard and rare decimal opcodes. There are also several emulators and projects.
There are several (better) photos on the web, but this one showing an open expansion. I also didn’t knew that it can also be combined with robots, cranes and parts for model railways and Fischertechnik.
I found high res brochures on this forum (this one 1983/1984)
DAS EXPERIMENTIERKASTEN-BOARD • Forum anzeigen - DAMALIGE WERBUNG
Another interesting kit (but not a computer) is this Video kit from Schuco/Philips with small oscilloscope. I have never seen that before. Maybe it can be used/reused for making a computer (or game).
Some more here
https://www.experimentierkasten-board.de/viewtopic.php?f=91&t=1476
Another video kit for use at a TV is Philips EE3023. At least someone can make a video game with it (probably Pong). More details also there.
Also of interest the Philips MasterLab MC 6400
Another, 2006 Kosmos kit (incl Windows tools) I never heard of before is this one
https://www.experimentierkasten-board.de/viewtopic.php?f=2&t=1676
Best site with illustrated part lists and manual is this one, this subpage for the 2nd video kit. On the left are links to all kits, though difficult to identify the manufacturers there
I can only speak for Germany, what about experiences/kits in other countries?
I always considered buying a CP1, but there are enough emulators. And the most interesting part (of most kits) is the manual.