My gym actually offers a weekly capture the flag game in a local park. It tends to be pretty well attended with about 30 people showing up every week.
Many people would not think about capture the flag as an aerobic game, but the truth is that you are running, jumping, crouching and crawling almost constantly. I always feel worn out in the best kind of way when I'm through.
And what I love is that this is a lot more fun way to exercise than simply riding a stationary bike or lifting weights. You quickly get lost in the game and forget that you are exercising.