I know it's not the same thing as I am yet to have my beautiful baby but have you considered buying something for the home? Just for now? There are small cross trainers or bikes which can fit in relatively small spaces and you could pull it out when you watch the TV or something, two birds with one stone. Plus you could keep an eye on them if they're in the same room.
Probably a naive idea but thought I'd try
Some gyms have creches, the one near me does, has your's not got one? A mate of mine found a posh gym/ leisure place once in London that had a baby gym too, yeah get them on the treadmill too!!LOL