First feature film to introduce the new Ford Mustang. Even though Goldfinger (1964) is commonly credited with this distinction, that movie wasn't released until January, 1965. In this movie, Annette Funicello's character, Connie, is seen driving the Mustang in several scenes. Note: This item is true only regarding US audiences. The Mustang did show up first in Goldfinger, which premiered in England and elsewhere in Europe on September 1964, two months before this movie was seen anywhere.
Toni Basil and Teri Garr, who appear in the clothing/modeling scene (of the four girls modeling fashions, Basil in the swimsuit, Garr in the yellow mini-dress), had a history of working as dancers together. They were also featured in Shindig! (1964), on which Basil served as choreographer.
The portrait of Uncle Henry, Aunt Wendy's late husband, is in the likeness of Keenan Wynn, who played Harvey Huntington Honeywagon in Bikini Beach (1964).
Tommy Kirk's singing voice was electronically enhanced.
Credited as Teri Hope, look for a very young Teri Garr, prominently featured in most of the major scenes. She's often just to the right of Annette Funicello in the dance and fashion show scenes.