1) go for a walk. If you do fast/slow/fast/slow... interval walking you can get a good workout in a short time.
Do you have a private spot you can do some exercises? If so...
Alternating weight-bearing with cardio exercises in short bursts. Do 5 minutes an hour and by the end of the day you've done 30+ minutes of workout:
2) Squats in different varieties. With no weight you want to do fairly high reps. If you have a medicine ball (just a couple of pounds will make a difference), you can add that when you're ready. Just make sure you're sticking your butt out and don't let your knees come out too far over your toes, and tighten up your abs. Work up to 40 in a row. You can vary this by raising your arms a bit when you squat (balance), touching the ground at the bottom of the move, etc.
3) Jumping jacks.
4) Lunges. I like to lunge back, rather than forward, easier on the knees.
5) Toe touches. Keep your legs straight and don't go too fast. This is cardio + core.
6) Push ups. Keep your core tight!!
7) Groiners. Get into a push up starting position, then put one foot up by your hand (on the outside), then switch legs. Do this for 30 seconds and you'll be breathing hard!
8) Bicycle crunches
9) Burpees with or without a pushup. Start in pushup position. Do the pushup (if you're doing them), then jump legs up to about a foot behind your hands, jump up. Then reverse. Start with a few and work up to a minute. Great all around exercise.
10) Stretching to finish up.