some apps I like:
interval timer AD
What this does is let you set up an elaborate interval cardio workout. So you set the warmup time, then the intervals and rest periods which can be repeated x times and finally the cooldown. I use this while I'm on the exercycle and I just follow the beats.
Push-ups - chest exercises
I use this for calisthenics such as pushups, situps, etc. Obviously you don't have to use this just for pushups. Instead of counting minutes like the interval timer, now I count reps. It has a workout routine but you can create your own custom routines with x number of reps, y number of sets, amount of time per rep and the amount of rest time between sets. It's the closest thing to a boot camp in your phone!
Possibly the best math app on Android. You can even do animated 3D graphs on it.
Brain Age by Best Addictive
challenges your memory and arithmetical skills
Memory trainer by Urbian
another well made memory game.