For me, Princess Princess would always be the band that signified my time in Gunma from 1989-1991 in the same way that the Yellow Magic Orchestra did during my 1981 summer vacation with my classmates and that TRF did in the first couple of years of my very long stay in Japan between 1994 and 2011. Those 2 years in the Japanese Alps were often filled with "Diamonds" and "Oh Yeah".
Princess Princess' 18th single "The Summer Vacation" from May 1994 was a disc that I picked up probably several years after its initial release since when I did come over late in that year to start teaching in the Tokyo area, I didn't hear anything about the band anymore. It was pretty much time for the Tetsuya Komuro（小室哲哉）steamroller with acts such as the aforementioned TRF.
Listening to "The Summer Vacation", it was very different from that more famous and more popular summer song that Princess Princess created in the late 80s, "Sekai de Ichiban Atsui Natsu"（世界でいちばん熱い夏）. That latter song was firmly planted in Japan and at that time, but "The Summer Vacation" seemed to take a time warp back into the 1960s and head over to the States. Vocalist Kaori Okui（奥井香）and lead guitarist Kanako Nakayama（中山加奈子）, the same duo behind one of their biggest hits of "Diamonds", took care of this song which sounds like a mix between an upbeat Tom Jones tune and a 1960s breezy American sitcom set in New York (the ending reminded me of "Love American Style"). It was actually used as an ending theme for one of Beat Takeshi's many variety shows.
The song is probably one of the few in the band's discography that wouldn't be anywhere near being called a rock song but neither was "Diamonds". "The Summer Vacation" peaked at No. 21 on Oricon. It may have been quite different from the usual Princess Princess tune but I could quite imagine the band in a convertible bombing down the Ventura Freeway while this is playing.
And hey, why not? Here's Tom Jones with "It's Not Unusual".