Workouts can be fit in anytime and anywhere. I have worked out in hotel rooms, pool side, and even bathrooms(yes bathrooms). If you don't have a long time to workout, you can definitely bust out a hard workout in 45 minutes or less. You can do this in the gym or wherever you are!
Here is a sample:
20 Squats, elbows to knees
20 Push ups
20 Reptiles, hold a plank on hands and toes, bring knee up to elbow. Repeat other side
20 Power Lunge Jumps
20 Stationary Frog Jumps
20 Jumping Jacks
20 Plank Rows, holding weights in a plank position, alternate rowing up(you can do this with a water bottle too if you don't have weights. Just alternate the bottle between hands instead of holding on it)
20 Alternating lunges with Shoulder Press(again, no weights use a water bottle or anything you can press over your head)
20 Bench(or coffee table) Triceps Dips
20 Skater Jumps(side to side, like a speed skater)
20 Mountain Climbers
If you don't have a skipping rope, just jump in one spot. You can always modify some of these exercises, example keeping your feet on the floor for jump squats, or stepping back instead of jumping for the burpees, especially if you have knee problems because the jumping can affect the knees.