I mean, chocolate rivers? Lickable wallpaper? And my personal favorite - the Everlasting Gobstopper. I love that word, Gobstopper. It's a safe bet that before Dahl came up with it, the word had never been uttered before in the history of made-up words.
When my daughter was in grade school, her class read The BFG. If you're not familiar with the book, BFG stands for Big Friendly Giant. Dahl gave his other giants in the book less friendly-sounding names like Gizzardgulper, Fleshlumpeater, and Meatdripper, all whom eat 'chiddlers' (children). I love his books just for the names.
In one scene, the BFG introduces the main character, Sophie, to 'frobscottle', a carbonated drink. They take turns swigging from the bottle and the BFG starts 'whizzpopping' around the room. The BFG has quite a vocabulary, thanks to Dahl. Words like 'lickswishy' (tasty), 'squifflerotters' (bad humans), and 'snozzcumbers' (something edible) sound perfectly normal coming from the BFG while talking with Sophie. It's really an art, using made-up words effectively in fiction.
For more about authors who have made up words, check out this article at Mental Floss. Funny that most of the list is comprised of children's authors!