In DKC3, when you enter Wrinkly's Save Cave, on occasion Wrinkly will be playing her Nintendo 64, as evidenced by the music being an Expy of the Super Mario 64 "Inside the Castle Walls" theme note note they are not the same pieces of music: compare the original theme here and the DKC3 version here.
Also in Wrinkly's Cave, sometimes she will be sleeping, and the background music will be the baseline from the "Jungle Hijinx" tune from the first game.
Bad Export for You: "Jungle Fantasy", the Japanese OST for the original DKC, contained seven arranged songs, all exclusive to the soundtrack. The OST was eventually released in Amercia as "DK Jamz", though out of all the arrangements, it only contained Jungle Groove, leaving out the other six songs (twoexamples).
Keep Circulating the Tapes: The original DKC trilogy was removed from the NA and PAL Wii Virtual Console services for reasons known only to Nintendo, meaning the only way to get copies of the game is finding either the original SNES cartridges or the GBA remakes.
What Could Have Been: Cranky was meant to be a lot nicer during development of the first game, as seen here. A giraffe animal buddy was planned, as well, allowing DK to climb the giraffe's neck to reach high areas.
DKC2 has some sprites of Diddy and Dixie mourning, which have been Dummied Out. They seem to have been meant for when they lose a bonus game, rather than the screen freezing in the actual game.