Like people said, biking is a great. You save money on gas, you get a workout, you get to where you need to go if you make the time for it. Your legs turn to steel and you develop a butt women like to look at.
Cardio running your dog out of energy isn't quite the same thing as a consistent hour's run, so you might want to give that a shot, whether on the streets or on the treadmill. The streets can be harder on your body but treadmills can lead to cheating. But they can also scale to give you a harder run than you'd find in your average locale.
Derpy as it sounds, hackey sack is good for flexibility, coordination and getting your heart rate up, when you play alone. It's low impact but if you're out of shape it can help you start shaving off weight. (I remember when I started at my current job, guys were huge into daily matches of ping pong. Maybe it's a testament to how out of shape I am, or maybe it's a testament to how simple things can give you a workout, but I was sore after playing at their level of intensity for several weeks until my body acclimated to the routine.)
When it gets down to it, working out for your health is about will power. Friends make that easier....but the real test is to go it alone because you've decided to make it important in your life. Do whatever it takes to cut down on the awkwardness of it. (And believe me, I know. Being a guy that walks into a weight room alone, powers through their workout and heads out is a little awkward. Then again, that's how you should be working out.) Music, books on tape, the right time of day so you have whatever privacy you might want....whatever.
And I'm not like, some fitness buff. I'm in pretty bad shape these days compared to where I was. But I've always found once I start working out, actually putting my heart into it....feeling better, stronger, slimmer becomes its own motivation. Like I said, I think that's the trap a lot of people fall into. They go at it half-assed, don't really see the results they want in the time they want with the effort they're putting out and they slip out of caring about doing it. That change, that transformation that makes you go "wow, I can totally body hack myself!" only happens when you a) really do a routine well and have good form and b) have the willpower to maintain it.