if you liked monte cristo, les miserables is really good! not 'fun' per se given the subject matter but despite the size and its reputation it's not that serious, as you say (it's actually pretty funny and the bits people complain about, like when hugo goes off on hundred-page-long tangents about the sewer system or the church or whatever, are some of the most interesting chapters imo). i haven't read it in a few years but iirc it's about the later socialist(? more left-wing anyway) french revolution, not the main liberal one. even if i'm wrong there it's still interesting

unrelated to anything you mentioned but one of my favourite non-heavy novels is the secret history by donna tartt (satire of new england academic elitism, very funny & beautifully written),