Sonic Adventure 2 is more of a footnote in the Sonic series history these days, but back in 2001 it was an event. Sega wanted to make a big deal out of the fact their hedgehog had reached 10 years old. There was a demo CD bundled with Phantasy Star Online. Sonic Adventure 2 even had a worldwide release - a rarity in the early 2000s.
So what did we get back then? What do you get now with the updated PC version? Sonic Adventure 2 looked really good for the time, and it's still a decent early(ish) 3D platformer despite the horrible camera.
Sonic Adventure 2 also had one of the best Sonic story lines: what if a "clone" of Sonic allies with Dr Eggman and frames Sonic for crimes against the people? It was a bit melodramatic at times, sure, but it took some risks without going too far off the deep end. There's a moment where Eggman makes half the moon explode with a single laser shot, which ends up as far more effective than any of his more modern plans. And is a literal moonshot!
It wasn't just the story that was really good, as Sonic Adventure 2 was (and still is) one of the better interpretations of Sonic in 3D. The 2012 re-release still has issues thanks to the camera, but Sega did their very best to buff out the imperfections and add polish.
And, of course, there's Chao Garden. You can raise Chao as pets with energy crystals and rescued animals from any regular level, then pit them against other Chao in races and sparring contests. I have to admit I spent an unhealthy amount of time raising my Chao, and would not be surprised if it becomes a time sink for you as well.
Sonic Adventure 2 was a very good game for the time and still holds up now. It certainly made Sonic's 10th birthday special.