Rudy's imaginary clover friend is named "Thidwick", a refrence to Thidwick the Moose.
The Mayor has a red fish as a pet, possibly a refrence to One Fish Two Fish Red Fish Blue Fish.
One of the portraits of past mayors was Dr Seuss himself (specifically, the one of Ned's father)!
The Other Darrin: The 1970 special is very inconsistent in voicing background characters. This is most notable in the "Mrs. Toucanella" number, where the birds seem to have different voices every line.