The collection of "Zen labs" apps for Android and iOS are a great option.
I suggest:
10k trainer (endurance)
0-100 push-up trainer (upper body)
0-200 sit-up trainer (core)
0-200 squat trainer (lower body)
There are several variations of push-ups, sit-ups, and squats. You're welcome to use any style you like or need.
My options for reference are:
Push-up: military
Sit-up: v-up
Squat: shiko