The Prisoner of Zenda seems an excellent choice, very well written and not 'heavy'. In a different vein, perhaps some of CP Snow's earlier books and the 'a dance to the music of time' series by Anthony Powell. Also any of Dorothy Sayers detective stories - Murder Must Advertise could be a good one as it introduces a more muscular Wimsey than one usually pictures, but my favourite is The Nine Tailors. None of these are classics or 'heavy' reading, Snow is harder going than the others, but they are well written and hold the reader.