Member Battle CDs and Battle Bingo
Most people probably skipped them but they are still probably my favorite side-content/minigames of any pokemon game ever
The battle CDs were puzzle battles where you had to do the perfectly correct moves to win the match in a certain turn limit and I'm pretty sure RNG was disabled. As a baby casual these were so difficult that I gave up, but they actually taught me the importance of moves that I thought were gay because they didn't do any damage (things like swords dance, thunder wave, reflect/light screen). I actually improved and began using these moves in my teams after playing some of the CDs.
https://www.youtube.com/watch?v=XZWZ5iY2xdIThere were FIFTY of these puzzle battles, and when I replayed them that one was the one that gave me those most trouble. The video is the correct order you have to follow, and if you do any other move but the ones shown you will lose
Battle Bingo was fun as a casual and still fun when I replayed all of them. You select from a grid/card of mystery battles and try to win as many as possible. Its 1 on 1 battles and you can use a limited amount of balls to catch an encounter, and the mons on each card are designed to lose to or counter different ones on the card.
https://www.youtube.com/watch?v=fYTO6_8XA5sNotice the effort they put in these shitty little sidegames that most players probably skipped. Compare that to the most recent MAINLINE games getting the full attention and maximum effort.