So, the gym membership is definitely an area where I am not mustachian.
I had a membership to 24 hour fitness, which is $99/year. It has a pool, which is about 13 yards long and two lanes (no, that is not a typo).
I got injured running distance (half marathons) and switched to triathlons. I didn't know how to swim, so I started swimming a lot. Very quickly the 13 yard pool got old (try swimming 1000 yds, you lose count very quickly).
After about a year, we finally joined the YMCA, 25 yd pool and 6 lanes. It was a godsend, especially later while I was pregnant. I did a LOT of swimming.
Anyway, I have two young children (7 years and 9 months). I've had a hard time getting out to work out the last few months...baby teething, not much sleep, still nursing the baby. But before the teething I did it in the early morning. We get up at 5 am. My husband gets MWF, I get Tues Thurs. On weekends we're flexible, but often I get Saturday for walking with my friends and he gets Sundays for tennis.
I also prefer to get exercise into my daily schedule. Pre-2nd baby I was biking to work a couple of days a week, and my Saturday walk was to go grocery shopping. I do really like swimming though, and when you are dealing with injury (or are hugely pregnant), it's a great exercise. I know I *could* exercise at home in theory, but I have challenges. My house is too small to have a separate room, and it's very difficult to get even 15 minutes without the boys bugging me to do any kind of workout. I used to be about to work out even on my husband's days. I'd get up at 5 with him and pop a video in (P90X, yoga, whatever). But then my older child was waking up when I did this (he wakes at 5:30 or 6) and he'd start bugging me. Really the gym is the best way for me to get in a solid, uninterrupted workout.
Plus the YMCA has a lot of family events and options. There are parents' nights out and my son has access to the pool, swim lessons, camp, kids' gym, sports leagues, etc. Mostly we just use the pool.