Any fitness regimen needs to incorporate a high level of cardiovascular exercise. Even if your primary focus is on weightlifting or resistance training, you'll need cardio to reduce your body fat so that your physique can show through. Just on its own, cardio is king for burning calories, detoxification and endorphin highs. Luckily, you have a lot of choices of how to add cardio to your life.
Get a bike. You likely had one as a kid and loved it. Why not have one as an adult? You might not get so serious that you ride city streets to work and back, but there's probably a good park or trail you can hit for a pleasant Sunday afternoon ride.
Get to know workout machines like treadmills. Also check out ellipticals and stair climbers. You might feel like a hamster on a wheel, putting in that much effort and not going anywhere. However, any gym will have these, and they let you workout indoors, in a lit, dry environment when exercising outdoors might not be an option due to weather or long nights.
Always keep an eye out for boot camp classes. Most urban areas are going to have one on the nearby calendar. These are high-intensity exercise sessions that last only a few hours or days. You probably have at least one friend that occasionally goes to these instead of working out regularly. Join him or her so that you can have some intermittent company in your fitness regimen.
Go dancing. It really doesn't matter what style of dancing you do, as long as you're active and getting your heart rate up. This is a great thing to do with your significant other, if they're not into exercising or workout differently than you do. Even if your loved one is not interested or in the picture, find a studio and just sign up. You'll make friends to dance with. Don't let not knowing anyone scare you off. That just means you aren't embarassing yourself in front of anyone you know.
Use your feet. This probably sounds too obvious to the point of being ridiculous, but think about it. If you live a largely sedentary lifestyle, just being on your feet more will do wonders to get your metabolism going. If large scale walking and running seem like too much, just get a pedometer and try to take 10,000 steps any given day. The difference that makes will shock you.
Now that you know a handful of the many choices of cardio exercise that exist, you should have no trouble finding particular ones to add to your daily life. Remember, any cardio exercise that gets your heart rate up for a set stretch of time is an effective cardio workout. A great rule of thumb is having difficulty carrying a conversation but still being able to talk. Pick and choose among all these to keep an interesting variety going, and change up as needed seasonally or depending on weather.