Welcome to Stiq Figures, where the sales data is after the break and the posts don't matter.
Sonic Adventure 2 is a prime example of mine – while I enjoyed sprinting through levels as Sonic and switching between casts in the game's good and evil campaigns, there was a surprisingly-compelling side attraction in the Chao Garden, a sort-of pet simulator. At the time, it felt like a fleshed out version of the Tamagachi I had as a kid, and I burned plenty of time bumming around the space and boosting my Chao's stats so it could compete in minigames. I ... may have also gotten a kick out of hurling Chaos as far across the map as possible.
Returning to SA2 in recent years hasn't proven to be as fun as my memories of it, but entering the Chao Garden is still a good time. While I'm at it, I should give a shout out Final Fantasy 8's Triple Triad card game too – for as awful at it as I was and still am, I've still spent hours losing cards to any character that's willing to play a round against me.
Has there been a minigame that's kept you enchanted, maybe even moreso than a game's main campaign? Help me acknowledge the side content that doesn't necessarily feel like side content, right after you check this week's Japanese hardware sales after the break!