Take it from first hand experience: definitely DO NOT deal with Bally's. For the 8 days that you are there you will be constantly pestered to sign a contract. If you do sign up for a membership, god forbid, you should pretty much consider it a life-time membership. Bally's is notorious for continuing to debit you even after your contract is up and you quit.
My father was nice enough to get me a year's membership back when I was in school. After the year was up (and my contract was complete), I notified Bally's that I wouldn't be renewing both in person and then in writing. 3 months pass, and I get a call from my father asking if I had cancelled my membership - he's still getting charged. I contact Bally's and they, of course, say they have no record of my cancellation. I kindly ask for them to check again and refund the money. They reiterate that they had no record of my cancellation. I asked them to cancel my membership on the spot. I leave thinking even if I can't get the money back, the membership is now definitely cancelled, right? Wrong... next month my father's account is debited again.
Long story short, this went on for almost a year. After contact BBB and other authorities (who could give a sh$t), I finally had to corner the manager in his office with a couple friends and watch him remove my name from the membership list.
Sad thing is, I am not alone in being a victim of Bally's. If you just run a simple search on the web, you'll see numerous unhappy/defrauded customers and several class action lawsuits.
So yeah, don't even think about this (even if it is free). Also, out of all the gyms I've worked out at, Bally's is by far the dirtiest and most dilipidated.
Edit: If you're looking for a good gym, try Equinox. Although it can be a bit too metrosexual and frou-frou for my tastes, the ones I go to usually aren't too crowded and they're always very clean. Most importantly, no contract shenanegans (yet...). I think they're having a 1-week trial this month...