I usually go to short stories, or old sword and sorcery novellas. For the former my go to stories are Lovecraft’s Cthulhu Mythos, Robert E Howard’s Conan, and Isaac Asimov’s Robots. For the latter I prefer Michael Moorcock’s Elric of Melniboné, Fritz Leiber’s Fafhrd and the Gray Mouser, and Jack Vance’s Dying Earth. If I’m feeling uninspired or experiencing a block, knocking out a few of these stories always sets me straight. They take next to no time to read and are great fun. I don’t get tired of rereading them.
Anything by Stephen King, for me. I also like quick-paced techy/sci-fi novels for when I’m in the mood for something easier. I’m a re-reader, so I will sometimes pick up something I’ve already read and enjoyed before.
Fun, feel-good fantasy stories, like Legends and Lattes. Bonus points if there’s a heist, a fun group of quirky characters, or well-built romance story.