In The Case of the Missing Rugrat, the residence the old ladies live at is called Grey Gardens, referring to a documentary of the same name in which a mother and daughter are rich recluses. Similarly the old ladies are rich but never leave their homes. Also as in the documentary, their house is in disrepair and quite dirty for people of such wealth.
The two elderly heiresses live at 1531 Selma Avenue. That location, in Los Angeles, California, adjoins Sunset Boulevard, which is the title of a famous film that also features an elderly and delusional and her German butler.
In The Case of the Missing Rugrat, all of the stores in the strip mall where Grandpa loses Tommy are puns referring to famous actors. There's Sidney's Pottery (Sidney Poitier), Mel's Books (Mel Brooks), Phil's Silverware (Phil Silvers) and Humphrey's Yogurt (Humphrey Bogart).