Gym tracking app?

[politicat]

I have a gym membership. I use just the machines. Free weights have too many variables that my iffy joints complain about if I'm not careful, and to be honest, my attention tends to wander at the best of times. I'd like an app that lets me track reps and time on those machines over time, and will prompt me to up my game. Bonus if it's simple. If it gives some sort of positive feedback (achievement unlocked), even better.

Any recommendations?