My friend was playing my Ruby version since he didn't have Ruby or Sapphire yet, and he said "this wild Numel looks like it's the wrong colour!" I took a look at it, and thought that this Numel with a blue back looked odd but it must be normal, right?! I had a little gaming book that had a list of all the new 3rd generation Pokemon, so we looked up Numel and saw that he was supposed to have a green back! We thought it might have been a glitch, and caught him. I think it was via Serebii that I learned what shiny Pokemon were. That Numel was the first shiny I found, and didn't find another one until ~ten years later when I found a shiny Sawsbuck in Platinum.
One thing I loved was venturing to the volcano area and running around, removing the soot from the long grass.
I also loved to put my character inside the "hot sand" next to the hot spring. I imagined being comfortable and toasty under it!
Another environmental thing I loved was the area to the South East of Fortree town. Loved the new tall grass, the puddles and the rain.