There's nothing like children's books for light, easy reading. And if they're by Terry Pratchett, whose idea of writing for children is to add stuff, so much the better. Diggers and Wings, the two later books of the Bromeliad trilogy, can also be read by a discerning adult as satire of the narrow views we sometimes take of our world. The nomes who escaped the destruction of the Store in Truckers now have to deal with life Outside. Grimma stays behind to help protect the quarry from the incursions of humans while Masklin leads an excursion to the airport in order to try to signal their interstellar ship.
Naturally, both their tasks would be made easier if they knew what they were doing, but what fun would that be? And an anecdote about tiny little frogs leads to this trilogy's name; the Bromeliad is to be read even if you think you're too old for children's books.
The Princess and the Goblin, by George MacDonald, is one of those children's classics that I missed somehow. It is told in a very traditional fairytale style, which is good if you like that, and leaves a few loose ends to be tied up in later books (which I also haven't read.) Good deal for younger folk, particularly little girls (though little boys might find a role model in Curdie.)