Walkthrough: Pikachu Valley is a new area added to The Alola Region, being just off of Route 4 where Piakchu tend to gather. There is not much to this area outside of the interactable Pikachu; however, if you scan the I Choose You Pikachu Event QR Code, you will obtain said Pokemon here. You can also obtain Pikashunium Z and Pikachu Outfits here.
I Choose You Pikachu - From Deliveryman After Scanning Event QR Code
Items:
Magnet - Near Pikachu Trailer
Pikachu Outfits - From Woman After Answering Pikachu Questions Correctly
Pikashunium Z - Speak To Far East Pikachu With Cap Pikachu In Party