18. Mushroom Kingdom
Head to the castle and do the really long cutscene.
After Peach leaves, head outside to meet her again.
As she's coming down on her umbrella, quickly run back inside the castle and re-exit immediately. She'll be on the ground without having to wait for her to float down.
Peach joins the party!
Head out the southwest exit (faster than southeast) and head to Tadpole Pond again.