Try this circuit workout 3-4 x a week. This can be done anywhere (home, vacation hotel room etc) as there is no equipment and minimal space required. So there are no excuses!
10 pushups
25 air squats
low plank hold (1 min)
wall sits (1 min)
The 4 exercises are to be performed in circuit fashion with no rest in-between. When all 4 exercises are completed rest 1 min and repeat the circuit a total of 5 times.