Sometimes I just don't have enough time for a longer workout so I do what I call the 20-4. It takes about 10 minutes. Here it is:
-10 squats (wall squats, sit to stand, or regular)
-10 lunges on each leg
-20 pushups - there are so many styles to chose from so I mix up the styles
-20 ab exercises of your choice. Right now I am doing bicycles and reverse crunches

Repeat the entire sequence. Check the Exercise Encyclopedia for detailed exercise instructions. Try the 20-4 and let me know what you think.