Two pieces of important gym equipment include a workout journal and stopwatch (interval timer). The fitness log lets you consistently measure progress. The stopwatch (or interval timer) lets you measure the third variable at the gym, time. Who did more work the person A that does 5 exercises of 3 sets of 12 reps in 20 min. or Person B that does 5 exercises of 3 sets of 12 reps in 10 min.? This is just one reason a stopwatch or interval timer is handy. Find more value in these by getting them
I use the Gymboss interval timer (http://www.gymboss.com/) and highly recomend it.
Alternatively any stopwatch that you have or get for cheap can work. Additionally, any easy to write in and sturdy book can be a good gym log book or journal. I use a hardcover composition book.