I constructed these myself. They are a wonderful way of helping clientele to stay on track and the logo will bring some attention to your business if it gets seen at a local gym. I try and send my clients out among the fitness world donned in as much logoed merchandise as possible. Shirts, hats, notebooks, bumper stickers, and water bottles are really good at displaying your brand at the right time, and to the right people.
As far as the actual log: have a space for time and date, exercises (make this area a little bigger; notes usually get added to each exercise based on individual preference), reps/distance, weight/incline/resistance, and be sure to put a special section for cool down and/or stretching so it stands out as a necessary component on its own.
Good luck, these are great little tools for client success and business marketing so, take advantage of both.
I make my own and customize each notebook to the client. I am sure you have good printer companies in your area to choose from. Have you checked out the large chain office supply stores? There are so many options and I still prefer to make my own; because, each client needs or wants something a little different.