Pokemon: Pichu to Pikachu

Both very nicely animated side adventures staring the animal companions of their respective shows. The MCs are just cameos in the background while you get a glimpse of the crazy shenanigans they get up to while the human characters weren't there. I regularly think back on both of these for just how fun and enjoyable they are. Not to mention both shorts were also made as tie-ins to watch with a feature length film.