Rubber is hilarious. There’s also squash, smash, hash (many words that end in “sh”); just about any noun with “licious” or “alicious” stuck on the end; lots of “bl” words like “blab” or “blurt.” Some phrases are fun: “spontaneous combustion.”
Fruehstucksflocken. Supposedly a German word for cold breakfast cereal, but since being taught it in 10th grade I've never had proof of its authenticity. Which leads me to believe that it is not a real word. Alas.
Also? Tazmainian, kazoo, and anything prefaced by the word "wee".