Someone upthread mentioned the Jim Butcher "Harry Dresden" books. I'm a big fan of those. They are fun and have a lot of unexpected twists. I also like the Patricia Briggs werewolf series (the more action oriented Mercy Thompson ones. I have steered clear of the Alpha and Omega series, which she says on her website are more romance-novel oriented). Those are both quick, light reads for the gym or the beach.
Another favorite, which I would consider "higher quality" fiction, is Neal Stephenson. "Reamde" was fanstastic. I think you have to have a certain specific background to love "Anathem", but I do. I even liked the over-long "system of the world" trilogy. I am bothered, however, by the way the big setup with a young Isaac Newton at the start never seems to have any payoff, or even relevance, later.
There were a couple of minor plot points, really just throw-away diversions, that get a quick, low-key reference a couple of hundred pages later. They cause a fun "oh, wait... oh, I know" reaction, then they are gone.
Actually, if I get that far, pretty much any novel in the last 50 pages is a book I can't put down.
[Edit - as cantunamunch reminded me, the right name is "Baroque Cycle." System of the World was one of the three books.]