You could try "Tabata" Workouts. Probably google it is best, but I'll try to explain. It's like you 55's kinda. You can use any excercise you want. Running, biking, push-ups, pull ups, squats, sit ups, sprints.............the list goes on.
Basically it's 20seconds of work then 10sec of rest. That cycle continues for 8 rounds which equals 4min. Throw 4 exercises in there and you have a great 16min work out. Or just 2 for a 8 min workout plus a jog or bike ride. You can download free timers or apps specifically for the tabata if you want
When you do the workout you do all 8 rounds of one exercise then move to the next. If you want to score yourself as you go it's the lowest # of reps at each round.
Example 8rnds of squat 17,17,17,15,15,10,12,12,
8rnds of sit ups 17,17,16,15,15,15,11,12,
8rnds of push ups 10,10,10,10,9,8,5,6
8rnds of pull ups 5,5,5,5,4,4,4,3,
You score would be 29
As I said you could do this with 1 or as many exercise as you want and do it anywhere.