Work opposite muscle groups. For instance, working your bicep and then immediately your tricep lessens your time in the gym while allowing an adequate rest period for the opposing muscle group.
When pressed for time always opt for strength training. You’ll continue to burn more calories post-workout than you would have just jumping on the elliptical.
Doing your cardio and ab routines first thing in the morning forces your body to use your fat as energy and helps create a furnace of a metabolism for the rest of your day.