Just roaming around the classic Spyro's hub worlds (notably, those in Spyro: Ripto's Rage), listening to the amazing music, and coming up with all sorts of scenarios in my head. My imagination just ran free, and it was fun and calming at the same time.
Speaking of Spyro, Ripto's Rage was the first game I ever completed 100%. To my younger self, that was quite an achievement. In that vein, the first few times I beat the E4 in Pok?mon (I can remember Silver and Emerald off the top of my head) got me pretty excited. Also, beating MD1 with a Skitty and a Chikorita, considering all the flying bosses, made my middle-schooler self pretty content.
Also, in grade 6, Pok?mon was pretty popular with the guys in my class, and we got to battle a couple of times. My best moment was when my Skitty managed to KO one of their Groudon. I mean, she got revenge killed pretty quick, but the bragging rights were still there. A lot of Pok?mon related stuff, really.
I played a lot of Sims, and I recall being pleased when I got several kids from my main family into private school. And when I maxed one of my Sim's career ladder, and a helicopter came to pick them up for work. Talk about an epic way to carpool...
Digimon World 1 was a lot of fun when I rented it from the local video rental place, playing a Digimon game was a bit novelty for me. Sadly, I sucked, but I remember it fondly none the less.
I could go on...