I suggest signing up for something that is so much fun that you will absolutely insist on carving out the time to do it, whether that means you will insist that your husband is available to watch the kids while you go, or you insist that your budget cover babysitting money.
Don't just look to gyms -- look to community recreation centers, city parks and rec programs and the Y. You can find a wide variety of
* Dance classes (belly, hula, salsa, African, flamenco, jazz, hip-hop) Advantage: You can often bring a nursing child.
* Yoga, pilates, and mixtures of the two
* Roller and ice skating
* Tennis and racquetball lessons and leagues
Also, it sounds like your gym doesn't provide babysitting. There are so many that provide it that if you like working out in gyms, I would switch.