My second entry will, just as aptly, be talking about the first proper expansion to the Pokemon TCG. This was the Jungle expansion. I don't own any cards from this expansion but I'm interested in buying sealed packs soon as well as some loose cards. It contained 64 cards which included some Pokemon, including third evolution stage mons that were not included in the first set. Notably, there's a lot less trainer cards. Instead, they've been replaced with more holographic variations of existing cards.
I really enjoyed the early pack art, not just the cards. The themes were simple, like Jungle and Fossil. It's different from modern expansions that have vaguely cool sounding names like "Silver Tempest" usually just paired with an image of a legendary Pokemon or something. An alternate idea I would have liked for early sets is if they named some expansions after locations in the world, like having a Cerulean City expansion with a lot of water-types and a Misty trainer card, or a Lavender Town expansion.