There's a hidden area in Castelia City where you can catch wild Eevee.
There's another hidden area where you some punk kids battle you, similar to
>>28590489Not many people seem to know about these.
Also in B2/W2, Sabrina is a recurring actress in PokéStar Studios. I also highly recommend you all go back and play those. Use the Bulbapedia Guide to get through it if you must. Just go experience it. You get tons of items, a "shiny sparkle" for your Pokemon, and there are TONS of neat details and shit in there.
At the risk of summoning Richard and his autism; there's an NPC in B2/W2 that says Lt. Surge is "Unovan". This is a nod to the fact that he was originally introduced as "American", but the USA has since been used as the inspiration for the Unova Region in the Pokémon World.
Oh, and lastly, B2/W2 has a reference to the Kalos PokéBall Factory. They don't mention Kalos by name obviously, just "PokéBalls are produced in a factory in a nearby Region" or whatever. Considering B2/W2 and X/Y were produced alongside each other, I doubt this was a coincidence.